Weekly. When I was single it was weekly or so for sheets, monthly for the duvet cover, and I rotate the pillows. I shower at night, don’t let the cat in my bedroom, and generally don’t have dirty sheets, so that’s more than enough.
Weekly. I’m going to the laundrymat weekly anyway because clothes need to be washed, so I go ahead and strip the bed before I go, and put the same sheets back on.
Also, even at 40+ I have a tendency to acne, and my dermatologist recommended years ago that I change sheets (especially pillowcases) and wash them in hot water at least weekly. It did seem to help, so I’ve kept the habit up.
Fortnightly. I sleep alone and have no sense of smell, so, whatever.
I selected “weekly,” though in practice it’s anywhere from 3-8 times a month. I don’t change on a specific schedule, just whenever they get manky. Since I have a 6 month old baby and a 3 year old who end up in our bed more often than not…yeah.
I am lazier about changing the pillowcases, however. I HATE putting pillowcases on. I have short arms and arthritis, and it makes me feel like kicking things.
Other - every 2 weeks. Every 8 weeks I flip the mattress. It has a cotton mattress cover that I wash then.
That’s the second option in the poll.
I chose “twice a month,” but it’s often once a week in the summer. We’re sweatier, the dogs are sheddier and dirtier.
Two or three times a year, whether it needs washing or not. A little more fastidious about the pillow cases.
Weekly. In cold weather, I have two flannel sets, I interchange the two. In warm weather, I have two cotton sets in which I do the same. Simple, and I have an emergency back up in case I spill something, which is rare.
Oh man, you’re my inverse. The limiting factor on my linens’ lifespan is when the pillowcases start looking oily/stainy/sweaty. All the other stuff could go for a month or two for all I care. 
Every three weeks.
I haven’t read the whole thread yet, but I wanted to comment on this. This has always been an issue in my household. Get a pretty throw-- there are so many of them everywhere now-- and cover the pillows and the top 1/3 of the bed with that. Even if the pillows are under a bedspread, a little extra insurance can’t hurt.
I put once a month, but that’s an average. In summer it’s around once every 2 weeks, in winter … longer than that.
Twice a month is not the same as every 2 weeks, except in February in non-leap years.
Most often twice per week but I chose weekly as that was the closest amount of time.
As I understand it, most Americans and Europeans do not not take a bath at night before going to bed. Isn’t it gross to get into bed with whatever dirt you picked up during the day and then be enveloped in that every night? I thought washing sheets at least once a week would be the default response, but I’m surprised at the number of people who go longer.
When they tell me too or I am having company. Less sarcastically usually when it works to fill up a load of laundry if I think about it. More when it’s hot and I am sweating a lot.
I’ve slept literally in the dirt after periods where the only daily washing was a wet wash cloth or babywipe rub down. My gross out meter is differently calibrated.
Sheets and pillowcases every other week maximum. More likely is every week. We have 3 dogs and 2 of them sleep between the sheets and they shed.
The comforters get washed monthly maximum, every other week minimum because the third dog sleeps on top and she also sheds.
Pillows get washed every 2 months or so.
I would probably still clean the sheets and pillowcases every other week even if the dogs didn’t sleep between the sheets because I love the feel and smell of clean sheets.
I wish I changed them weekly, but I just don’t get around to it. It’s probably closer to monthly.
Doesn’t anyone vacuum their sheets? I used to do that when the bedsheets started feeling gritty and it gave them an extra week of life before I had to wash them. I used a hand vac. I figured if it’s good enough for the sofa, why not the bed too?