I have long straight hair. I wash it when it starts to look flat - which is every 2-3 days.
Or if I get sweaty/dirty.
Every 2-4 days. Usually 2 or 3 days. I washed it more often when it was shorter, and I can get away with 4 days more frequently when it’s longer. My hair also gets oily very very slowly. When I was a kid, I washed my hair once per week.
I wash my hair almost every day. The only time I dont is (sometimes) the morning after I have a day off. This is because I work at Home Depot. HD is usually a very dirty store but my store is almost 20 years old. It is VERY dirty. When I leave there, my arms are brown (I’m a redhead and my normal skin color is typical-almost translucent). This is even after I wash my hands and arms at least once an hour. I can only imagine the crap that is in my hair if my arms get that dirty even with constant washing.
My hair is very fine but very thick and very straight but not lank, not oily or greasy. It is also about 5 inches or so past my shoulders. I’m happy to say that my hair looks good no matter what I do (or dont do) to it. I can go 4 or 5 days without washing it and it still looks the same as the day I washed. I frequently don’t brush it and I never style it. So, in my case, the constant washing is simply because of work.
I can get away with washing it every other day, but I like waking up to a shower every morning, and it feels weird if I shower without washing my hair. I use a pretty mild shampoo, so my hair doesn’t seem to be suffering from this.
I wash mine whenever it starts to get, by my reckoning, kind of gross. Maybe once every three to five days. After which my hair goes all fluffy and crazy. I am, however, a grotty male uni student with hair about halfway down my back, and you probably shouldn’t take my advice on this.
Short answer - once a week, conditioned about every second day, though.
Long answer - I’ve got a routine.
My hair’s bleached and dyed with semi-permanent dye, which washes out a little every time I use shampoo. On top of that, it’s dryish and fine and curly. My hair likes to be conditioned, but not so much washed.
It does get conditioned every other day, though. The night before, I apply powder to my scalp and bangs to sop up the oil. The next day I shower and rinse out the powder and add conditioner and rub it in like shampoo, massaging my scalp.
This gets out enough of the oil, no drying effects on my hair, and no wrecking my hair dye. 
It feels like a soft bunny when I pet it.
Once a week or so I’ll wash it anyway, but I think this is more a byproduct of me having so much freaking shampoo and feeling like I have to use it than actually needing to wash my hair. I washed it today, so now it feels horrid. Hehe.
Every other day, so I don’t damage my hair. After that, it goes oily so I have to wash it.
When it’s dirty (usually about every 4 days) unless it’s the Og-damned winter and the heater is off. In extreme cases i’ve been known to heat water in a kitchen pot, old style, but it’s a bitch.
Me, too. I will skip “doing” my hair a few days per week as long as I don’t have to go out, but I have to wash it everyday. The blow dryer and curling iron are wreaking havoc on my ends. I’m blonde, and dirty blonde hair is a total gross-out for me.
That’s what my hairdresser said. She said to stop using the Pantene conditioner and to use baby conditioner, but I don’t think they make such an animal, or if they do, they don’t sell it at Target.
Johnson and Johnson.
I want to add to mine - I never use styling products, and rarely use a hair dryer. I believe those two things contribute greatly to drying out your hair and scalp, maybe even more than washing it with mild shampoo daily.
Every day. It looks fine if I skip a day, but since I have no sense of smell, I’m kind of hypervigilant about hygiene.
51/female/thick, shoulder length, highlighted
Every day–I couldn’t stand to not clean it every day.
I use Suave shampoo.
Rarely blow dry, and until recently I only conditioned maybe once a month, now it’s more like once or twice a week. (I swim a lot.)
I always sneered at the whole conditioner thing until I discovered Scruples.
That stuff really lives up to the hype.
Every other day. I’m afraid I’m a hairspray junkie (Hi - I’m VCNJ~ - hand over that can of “Shaper” and no one gets hurt!) and it really kills my hair. I also condition like a madwoman.
VCNJ~
Every day because it is short and gets smushed while I am sleeping. I have to wash it to recomb it into shape.
When it is long I could go maybe one day without washing it, but would have to put it up. It looks a little flat because of the minor greasiness.
I have thick naturally wavy hair. My mom washes hers every few days as it is wiry and dry.
Once a week, because at that point my scalp is dirty enough that I can’t put it off any longer. I think my (dry, thick, curly) hair would be happier with, say, once a month, or perhaps twice in a lifetime, so I have to condition it into submission afterwards.
I guess the system works. I work at a daycare and the little girls are always asking to pet my hair 
Daily. Oily hair, plus gym time equals must wash daily. If I shower twice, though, I only condition the second time, no shampoo.
Aww. Reminds me of when my parents lived in a condo complex (50ish folks surrounded by young families) and the little girls would come by asking if she could come outside. She would sit on the stoop as they brushed her hair.
I never use shampoo. I haven’t used it in over a year. I have the most tremendously dry scalp and have found that conditioner alone keeps my hair clean and scalp moisturized. I condition my hair in the shower every other day. My stylist says my hair is in the best condition she’s seen it.