I had an ACL tear of my own a long time ago, before I ever started BJJ. I'd take Brandon's advice, and do what they tell you for rehab, plus don't worry about coming back too soon. You WILL get frustrated just sitting around, but you have to understand that these injuries take time to heal. Also, make sure you tell your doctors that you train BJJ so they know exactly what you want to get back into and give you a proper time frame of rehab.
I've had some nagging knee issues recently as well, and it's always in the back of my mind mostly because they haven't been MRI'd or required surgery. Even though this is a serious injury, you're lucky that you have surgery and rehab to fall back on so you know exactly what you need to do to heal it.