My knee injury, in which I completely tore the medial collateral ligament and both cruciate ligaments, is in no way comparable to yours. It ended up taking me about a year. Hopefully your meniscus problem will take a lot less time.

Talk to your physician and your physical therapist. You'll have to explain in detail what your activities involve, unless one of them is also a backpacker. They may be able to give you a guesstimate, but they may want to wait a while to see how you respond to the post-surgical therapy.

While you need to work hard at your PT, don't rush the backpacking. Between the pack, the irregular terrain and the downhills, that's a lot of strain. The PT folks will have you on a bicycle a lot. Make sure the knee is healed and that you've built up the supporting muscles first, and then start slow and work up. If you haven't used trekking poles, start now. Work on lightening up your pack weight, too.

The only thing worse than that "arthritis" tag is when they tell women that "women are prone to knee problems and there's nothing we can do about it." I didn't have that diagnosis, since my lower leg was flopping around with no control right after my injury, but both my daughter and daughter-in-law have been told that!