Here in southern California its year round, but what about places where it snows?
If you hire a gardener to do it, what do they do for work the rest of the year when the grass isn’t growing?
Since there’s no possible correct answer to this, let’s put it in IMHO.
samclem
Here in WI, late April (our lawn already needs a cut) until August (when the sun fries it), and then maybe a few times until late September. People who actually water and care for their lawns probably mow until late October. We have about 2 acres to mow, so the sooner it dies, the better.
I’m gonna guess they plow snow, do maintenance, etc. If it’s a retired guy, maybe work for a school district driving bus or doing maintenance. Around here it seems we have a lot of jacks-of-all-trades.
About 9 months or so. You can usually stop mowing around June and start up again in September or thereabouts.
I did mine for the first time of the year last weekend. It should really have been done about 2 or 3 weeks earlier, but I never found a dry day when I wasn’t at work.
So say late March/early April until probably the end of October or early November - that’s seven or eight months of the year. If we get a hot summer, the grass tends to go yellow and more or less stop growing, as I’m on sandy soil, and hosepipe bans are frequent (I could never be bothered using a hosepipe to keep the lawn green anyway!). That’s southeast England, BTW.
We very rarely get snow here - maybe half an inch lying once a year, if that, and usually gone in a few hours. But it usually gets cold enough that the grass stops growing for a few months in the winter. This winter just gone was quite cold and dry, but before that we’ve had a run of very mild and wet winters, and the grass has kept on growing, albeit slowly, for virtually the whole year.
Southern Ohio here. I usually mow for the first time in mid-March, and for the last time in October.
With the recent lack of rain (Melbourne, Victoria, Australia), I think I cut my grass about once a month over last summer.
I probably won’t have to cut it until Spring (September) and then if the rains still fail to fall, it will still be only about once a month until next Autumn (March)
Southern Michigan. Generally my first mow is in mid-April. In the spring it’s mow every 4-5 days until usually June, when it gets down to once a week. By the time August rolls around it’s generally about 1 1/2 weeks between cuts and the last cut is usually right before Halloween. I have mowed as early as late March and as late as Veterans Day.
The earliest our lawn needs cutting is sometime in March or early April, depending on the rainfall. There’s usually a dry spell in July or August where you can go almost a month without mowing. And the grass (and leaves) still need lawn care as late as November, again depending on that year’s rainfall.
Our lawn is shaded almost completely so the grass grows more slowly than other yards, and I’m no big stickler for yard work anyway, so I cut it on average every two weeks during the period when grass is growing.
This year, it could have used a trimming in late March, but due to our being out of town a week or so and there being much rain, it was just this past weekend before I got it cut the first time. It definitely needed it!
Mid-April to mid-October. This is for Bermuda grass in Georgia. Fescue has a longer season - maybe an extra month on each end.
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Don’t have a lawn. Well, we did plant some grass for the dogs in our side ‘yard’, and we might weed wack it once a summer, but we don’t even own a mower. Everything is just pretty much wild. Rocks, trees some grasses and wildflowers.
Works for me. Had 5 acres to mow when I was a kid.
The trade off is that I have to plow our driveway usually at least once a week for about 6 months.
Eastern Ontario:
Mid-May to Mid-September: starting with a couple of times a week and tapering to once every couple of weeks.
Gardener? Yeah sure, along with my maid and butler too. 
Cleveland here. I mowed first on Easter (coulda mowed earlier) and now I’m playing catch-up trying find a dry day when I’m home to mow.
Like others have said, we go to about September, and maybe once in October to grind up some leaves. Actually when I bought the house, I mowed the first week of November to grind up leaves, as we had a very mild winter.
My parents’ lawn guy is their plow guy, BTW. Most landscape businesses up here plow in the winter (they have the trucks, so why not?) but it is quite a pain for them when there’s not much snow. My dad’s guy offers contracts or per-plow basis. If you do a contract with him sometimes it ends up costing YOU more per plow, and sometimes it ends up costing you less.
Zero. That’s what I have a husband for. (Oh, like he’d let me touch the mower anyway!)
The real answer: We haven’t mowed yet this year, though a couple of our neighbors have. Usually mid-April to mid-September. Sometimes it’s so dry in July or August that there’s no need to mow.
Late February until Thanksgiving, most years. You mow a lot less in high summer, but you still gotta cut the weeds. I don’t water, which means I can go a lot longer in summer, but I still have to do it. I mow a few times in the winter, too, when the weeds get too big.
Ah, joy, desert landscaping, no grass a’tall. After years of mowing acres of the damned stuff, now we just enjoy the cacti, shrubs, trees, all in blossom now.
Excuse the gloat.
Oh, yeah, no need for a snow blower either. 
I’m still waiting to cut the grass here in interior Alaska. I’d guess it’ll be another week before the grass is green enough to cut. We can stop in August. Four months.
Like others, I don’t know anyone who has or is a gardener but I’d imagine there wouldn’t be many. Most people wedge all the fishing, camping, orienteering, and everything else outdoors in the long warm days before the jackets come out again and the sun disappears.
I pay an association fee for the grass to be mowed for me. They have mowed once this year so far, and will probably mow until October-ish depending on water/temperature. Often August is too dry to require much mowing. The same company that mows does snow removal, too. With the same trucks and trailers as they bring the mowers with.
OOOh, a question I’m supremely qualified to answer, being the sister of a landscaper! Here in Northeastern Ohio (as many have already stated,) the lawn-mowing season is April through late October. My brother does not do snowplowing, as many other lawn care professionals do, so here is how he spends his winters. When he was single, he used to travel, especially to visit me since I lived in Colorado and Georgia back then. Now that he is married, and I live three miles away, he stays home, catching up on his reading and computer time and healing up from all the physical strain. Once he had kids, he spent the winters taking care of the twins instead of them going to daycare. Now that they are in high school, he does the cooking and home repairs and reading and schlepping the girls around to all their sports stuff. He now owns the company, and he attends seminars and stuff to keep up his certifications, and does the paperwork and the taxes and all that.