I don't know of any rule of thumb, but I don't see why anyone would be concerned about forgetting songs they don't like enough to preformm often. Of why anyone would be concerend about some songs sticking for 30 years and others forgotten. The songs you love enough to do over again for years, are the ones that give your concerts a pattern. Something a long time fan can fall back on, trusting that you will do some favorite from that list, while the rest give the concert a new flavor so that it isn't the smae old thing that gets boring. BTW, if you ever make it big, the songs that got radio time and you are remembered for you can never forget. You will be forced to sing them, so you better like them. (ha, the idiots in the real world rarely make a song worth keeping popular) Which is one way you can concole yourself after a few years of obscurity. "Well at least I don't have to remember that old thing anymore, I can't belive I ever went through the effort of remembering it, much less performing it."