Theoretically, forever as long as the main plants remain loved and healthy. I don't think there is any initial genetic degredation anytime soon, I remember reading about a guy who exceeded 9 generations from the same original mother with no noticable loss ... that genetic route is still continuing. I don't mean all from the same mother, I mean the original mother was cloned, that clone was cloned, then cloned, then that clone cloned etc.
As long as you continue health and love, theoretically its not something to worry about.