I keep my music under 7.8 GB or whatever the capacity of the iPhone is once formatted. If it goes over i delete some old stuff, generally theres a few things i can delete without missing too much. I used to keep all my music all the time, then I had about 40GB, but i decided that was too much and i never listened to half of it. I boxed up the CDs, and deleted it. It would take hours to re-import it now.