2Pak, arthroscopies are easy today. I've been busted up and number of times. Three major surgeries on the left and no ACL on the right knee. Lost both ACL's years ago (ski racing). A year ago Jan., I was hit by an idiot crossing a ski race course. Split the left tibia in half... lengthwise, which split the meniscus in half as well. After five surgeries, (mostly due to infected metal in the leg), I'm 99% fine but I work at it nearly every day. Played two hours of tennis last night. I still ski / race without a brace
Your "down time" should be minimal but depends on your age, fitness, attitude and surgical procedure you have to endure. I'm 51 and there is no substitute for fitness. Weight training for your knees is imperative. You'll get some in PT but they are not trying to get you in shape, just not to hurt you. You have to tell the therapist what you what. If you really want to come "back," get strong. Your muscles can compensate for allot. You have to know your body. You should be able to tell the difference between pain you should work through and that which is causing damage.
Try to stay off the oxy and the like. Very addictive and you won't like it when you stop. Ice and advil work great. Take advil before your exercise. When you hurt... move the knee! Get the blood going. As soon as the Doc allows, get on a life cycle (not a bike, don't risk the fall until your knee can withstand one). Mostly, don't give up! Good luck.
Greg