I progress the weight until a goal, see my numbers above. I hit that goal this Saturday. I'd like to progress the weight some more until I fail. Front squats and back squats I will keep progressing because I can still complete the reps I assign to myself. when I do fail, I will switch from ME front squats to dynamic and dynamic back squats to ME. Starting at a weight that I can easily handle working up to a 4-8 week program that I might fail at (progressing 5*10 lbs per week). My OH squats are pretty maxed right now and I might just drop those down a lot real soon.
Your last question. You should answer that yourself because you will know pretty soon what you can and can't handle as far as volume is concerned. Just see how you recover, how your body feels etc.
I'm happy to hear that someone else does legs 2 or more times a week like myself.