I don't have an iPhone yet, but am hoping for a 64 GB model this fall.
My 64 GB iPod Touch is nearly full at the moment with only 2.9 GB free.
Obviously I don't listen to all my songs or use all my apps in one day, but when I do listen to music, I listen to it on shuffle, and I just like having all of my music there.
47.7 GB Music, 7.5 GB Apps
There is a good amount of kids music and apps on my iPod that won't be going on my iPhone, so take away about 5 GB for music, and possibly half of the apps.
Maybe that'll clear some space for some music videos. Or, just gives me room to expand my music library.