Is it important to take each set to failure, or just the last set? Also, is it beneficial to only go to failure on some days and stop 1-2 reps shy on others?

I had surgery 2 months ago & it kept me out of the gym. Definitely lost a lot of size, and I look flat as hell. I've been doing bodybuilding style training for 5 yrs & built a good foundation of size & strength (Bench 275, Squat 400, Deadlift 500+). How long will it take to start looking like I lift again? I got the OK from my doctor to get back to lifting. What are some of your observations when someone lost gains and regained them?