Smaller circles help them gain strength. I would work them up to the small circles tho as it can make them sore if you put them right into it. Walking and trotting initially and gradually work into a lope. I would think about a month if you work almost everyday at it. I would mix the exercises up and long trot as well. 

I just recently visited Oakridge Vet hospital for the same thing with one of my geldings.  The vet did the stifle blister and he also had me give my gelding a round of estrogen which is also suppose to help tighten those ligaments in the stifle.  When i was asking questions, the vet said he did not know that "hill work" helps the horse...and he mentioned that it might actually be too hard on them, especially at first.  

Ground poles spaced closer together will encourage a horse to lift their leg higher to step over thus working the stifle joint more than poles spaced further apart.  (I went as far as putting poles in the turn outs so the horses have to step over them coming to get food and water and i also created a "step over" in the horse's stall that they have to step over to get into their run.  I figured the more they were doing the work on their own, the faster the results, lol.)

So, in addition to the ground poles, lots and lots of long trotting every day.  I also pulled my gelding's hind shoes, stood him more upright, and squared his toes on the back.  This seems to have made a huge difference for him.

And if your vet didn't mention it, be sure not to give your horse any anti-inflammatories.

Lastly, there are exercises and stretches that you can do with your horse to target the stifle area.  I have incorporated the "tail pulls" which i will do while my gelding is eating.  * Info on that can be found on Youtube.

Best wishes for your horse.

I've had it done and I dont have hills either. I just trotted and loped alot. Fixed her right up.

Why do you not give any anti-inflammatories?

The internal blister catching stifles is almond oil and iodine injected into several sites into the patellar ligament. You don’t give anti inflammatories because you want to create some scar tissue to shorten the patellar ligament. What causes catching stifles is the ligament (is a little lax) gets caught/catches over the head of the stifle. It’s very common in young geldings. 

They also need exercise to keep from creating too much scarring. Whether you’ve done the blister or not, exercise is very beneficial for catching stifles. The more sedentary they are the more frequently they’ll catch.

My gelding has weak stifles as well.  I'm doing the "tail pull" exercises and trotting cavalettis.  I'm also jogging in small circles and backing him up more than usual.  Keeping my fingers crossed that this helps.  He's still working really well at the barrel races so I don't believe anything is hurting.  

I have one with weak stifles, normally she's fine with exercise 3-4 days a week. With the weather they've been in the stalls for a month with limited access to get to the indoor because it's been an ice rink outside. Hill work is good, walking and you're only supposed to start with small inclines. 1-2 times a week and building from there, but always at a walk. Even if you have a ditch on the roadisde, use that as bst you can. Cavalettis work better than ground poles, try to elevate the poles. We don't have any poles so I use shavings bags and just set 3-4 on the ground agaisnt a wall and make her step over them. Lots of long trotting is also good, straight lines preferably more than tight circles. Once you build up more stength you can do more circles. Backing is good too, when I'm leadinging her I do a lot of stopping and backing. I try to make her back up at least 50 feet a day. Hers were loosey goosey 2 weeks ago and catching & now they're staying in place and she's moving fine. I've been working on her 5 days a week, jsut lunging because I don't ride in the winter. She's been sitting since mid October, which is how I found out she had weak stifles. She's never sat that long since I've owned her so I've never known before. Now that I know she won't ever just be sitting aroudn again unless its required for an unjury,