How long after TKR will it continue to hurt to do stairs?

At nine weeks, your muscles and tendons that were stretched and stressed by the surgery are still recovering their tone and strength. Even if your flex range is very good, you'll have to keep on rebuilding muscle strength by having your new knee singly support the full weight of your body to move it up and down, as when using stairs. There are exercises for this. I hated these at first because they were uncomfortable and hard to do at at the start. But they were the best home exercises for regaining knee stability, done 2-3 times a day at 10-15 reps each direction.

Stand with your feet together on the last stair step down, facing the wall, hands on the rail or flat against the wall for support, TKR knee on the up side of step. Bend your TKR knee and try to tap the floor below you with your good toe before bringing the good foot back up beside the TKR foot. You don't have to actually touch the floor below (or a stack of books or board to make it easier) at first, just make your knee do the work of lowering and lifting your body. Do this also to the front, facing down stairs tapping your good foot down to the front, and then another set facing up the stairs tapping your good foot behind you and down.
I think I was given this exercise during the last two weeks of PT, and I continued doing it at home for another month until I had no more issues or difficulties going up and down stairs. I still do it once in a while to check on my knees.

I'm 9 months out and stairs hurt every day, every time. I exercise 3x week at a gym and one of my exercises is the Stairmaster to get some cardio after lots of stretching and prep. The knee inflames and takes till the next morning to calm down w ice and NAB. The other 4 days I walk or bike ride & the knee inflames after either exercise so it's just a question of how much pain I'm willing to endure to maintain some fitness. FYI, I'm 5'10" 172. If I gain any weight, knee pain gets worse. Right after the TKR, the pain was so intense I couldn't eat or sleep so my weight dropped to 165 and then the pain was less. Based on my observation, weight is a major factor in pain generation from this procedure. Originally, I intended to TKR both knees but fortunately, those plans changed. The non surgical knee is being treated with Hyaluronic acid which is providing some relief but after 9 months and endless PT, I achieved NO material gain from TKR. My unofficial survey says thin people who are sedentary will benefit from TKR but if you are not thin, and/or you intend to dance or exercise, my experience has been relentless pain and remorse. Sorry.

Thank you for your comments and my thanks to Mayo for organizing this forum so people can share their experiences & learn from one another. I've always been active, my entire life, used to play competitive racquetball. For two weeks in Feb I avoided the stairmaster but that did nothing to mitigate the pain from a flight of stairs. To gauge the impact of weight I climbed a flight of stairs and then repeated wearing 5lbs of weight around my waist. I found the diff in pain to be remarkable. A better exercise alternative is swimming but I've come to accept that pain will be with me bec I refuse to give up exercise. If I can lose more weight it should get better. Eventually, the technology will improve but I'm just thankful I didn't TKR both knees. My suggestion to anyone reading is to explore the alternatives which include, but are not limited to, loss of weight, Hyaluronic Acid, PRP, stem cell, cortisone and dietary changes. If I had it to do over again, I'd sooner go to a weight loss Boot Camp for a month than endure the grief that followed my TKR.

Mr Nomad, I totally agree with you about getting both knees replaced. I have had so much trouble with my knee replacement and all the people I talked to before my surgery said there would be no problem, yeah right!! I had my TKR on 2-19-24 and started PT right away. Could not get my knee to bend past 70 degrees so had a manipulation done on 4-12-24. The surgeon said he bent my knee to 120 degrees, but when I went to go home and get into a car, the knee would not bend any better than before and it was only a half hour later! A PT came to my home immediately and could not get it to bend any further either!!! I have been keeping up with PT in the hospital, but not much better at all. Will see my surgeon on 6-10-24. My knee is much worse and painful that it was before I had it replaced!!!