I'm getting an arthroscopy to repair possible torn meniscus, how long will I need to wait until I can exercise?

Depends. If your meniscus is amenable to repair, you will usually be non-weight bearing for 4-6 wks. Early therapy will focus on range of motion. Follow your surgeon's instructions.
It depends. After meniscal repair, you need to be nonweight bearing for 6 weeks, then gradually increase weight bearing as tolerated. You'll particfipate in a supervised rehab program for the first 3 months after surgery. After 12 weeks you can do low impact exercises. If you just have a simple partial meniscectomy (debridement), you should be able to return to activities in 10 to 14 days.