there's no such thing as an "average century".
If you're thinking about a 28+ cog, then you really should put a triple on. I cannot fathom the huge friggin' gaps on a 13-28/30 7-speed.
If you love the frame and it fits well, then it might be time to upgrade the entire drivetrain and shifters. If you don't love the frame, don't spend too much money upgrading the bike, get a new one that fits better.