I have signed up for a 21 day retreat at Wat Doi Suthep, but now I'm thinking of cancelling because I'm worried that three weeks of meditation would destroy my already bad knees. Does anyone have experience with retreat centers that have/allow chairs, and offer retreats of around 3 week duration?

Hey N A,
I went to Wat Chom Thong 6 weeks ago and they had chairs available.
Since Wat Doi Suthep follows the same tradition maybe they also have chairs there, did you ask them?
And if not, just go to Wat Chom Thong which is very close to Doi Suthep.

Adrian

Hi Adrian,
I hope you don't mind, but I've sent you a message asking about your recent trip to Chom Tong. Apologies for jumping into this thread.
Mo

Thatagata Meditation Center in San Jose, CA has chairs that are typically used for "resting (10 mins max)", as a sign says. But my knees were destroyed on about day 7 of a 10 day retreat and they allowed me to retreat to a chair. 

I'd advise asking for the option of using one in your retreat application. A few yogis were seated with access to their own chair.

Great center. Mahasi style.