Audrey, I am so sorry.
There is just not an answer to the "how long" question, each case is different. With our boy Charlie, who we lost to osteo at age six, the time was very short. Amputation was not an option for Charlie either. Others have had more time, maybe even months instead of weeks. What I can tell you is to try to take on the here and now that Asia lives in. Asia only knows how she feels right now, she is blessed to not know the future. As her human mom you do know, but try to not let that knowledge take control. Be in the here and now with Asia and spoil her, take pictures, enjoy ice cream and burgers, be in today, not tomorrow or next week or next month. Because Asia only knows today, make today count. Be sure that you have advanced pain meds on hand for when she does need them, don't wait until she needs them. Tell her vet that you must be prepared for when the Rimadil doesn't work. I would request Tramadol and morphine now, it seems that invariably pain bears down at night both in dogs and people. Because you love Asia you will know when it is time to let her go in peace. Until then take one day at a time or even one hour at a time. My prayers are with Asia, you and Bob.