Hello, and welcome to the forum!
I'm being boarded for my knee as well (severe meniscus tear) and know how difficult it makes getting around, and how painful a knee injury can be. It took me about 6 weeks (43 days, to be exact) between my packet going out and getting my results back. However, the results were terrible and now I plan on going to the formal board.
The amazing amount of information here helps with the worry. You get a much better sense of what laws have to be followed, how the process works, etc. through this site. Not to mention having your questions answered by people who have legal expertise in the field, and from people who have been through it themselves.
I highly suggest checking out the Conditions & Ratings section of this forum. There are stickied threads at the top from the VASRD (Veteran Affairs Schedule for Rating Disabilities). If you can't find what applies to you, or can't get them to open, I'll copy them and post them here for you.
Good luck!