It would be odd for a young dog to be put on rimadyl long term for something that could be fixed, like a luxating patella, ime. Has your vet mentioned any other treatment options?
All the people I know with dogs that have patella issues have said their vets recommend surgery sooner rather than later to avoid arthritic changes - if she's already in pain then it sounds like you might want to explore all of your options before just taking the rimadyl. Perhaps even a second opinion?
Erin, Ziva, Kestrel, Aerten, and Snipe
Always in our hearts: The Amazing Maggie Mae