This is probably a dumb question . I know many recommend hill work to build strength in the hind end, but how much hill work per workout do you do? Do you go up and down for a certain length of time, or up and down a certain number of times? I know it should be introduced to the workouts gradually, but I want to know I'm doing it enough that it's beneficial and not too much to where it causes more harm than good.

So I'm currently rehabbing my mare after lig snip for KS and splitting her stifle ligaments twice, so hill work has been important for me the last few months. I am lucky that my barn is up on a hill and we have a ravine next to it. The hill the barn sits on I would say is 75 feet high, but its gradual. So the drive is a couple hundred feet long. And then the ravine has varying sections of height and steepness. So I started just doing the driveway, walking it 2-3x a day for 3 days a week. And then I started to slowly add in shallow sections of the ravine that were steeper. We have a spot thats about 8' high that they have to do a decent climb. And the ravine is at the bottom of the hill. So I'd do that ravine spot and then do 2x the driveway. And I just kept building and mixing. She still does them 3x a week, I do them while she's cooling out from her hour workout. She's up to a section of ravine thats about 15' tall and steep, always done at a walk. I have her go up that 4x and back down a less steep way. Downhill is very hard on stifles so I find the least steep incline spot to go down, sometimes zig zagging like a mountain road. I also make her back up the driveway hill for about 50 feet in total. Eventually I will have her start to trot up the driveway hill for some extra boost. I think the amount you do is going to be dependent on the hill you have available. When I'm done with her official rehab I will still continue the hills 3x a week 

In my research, you would likely be the most worried about soft tissue injury in this case. Similar to an approach on working in deeper ground. The trick to avoid injury in just starting with hill work when not previously done before is gradual. 5-7 minutes at a time a few times a week. Fatigue in this case is the enemy. Avoid fatigue of the muscles (which will avoid strain on ligaments and tendons) until your horse is well accustomed to the work. Also ensure your feet angles are in proper condition so as to not put undo stress on tendon/ligament/muscles. 

SO in the beginning stages of adding hill work to your fitness routine, use it supplementally. Do NOT over do it.