Not every game does it, and they store various amounts. I currently have 8 games which are taking up 9202Mb of disc space, so I still have 45Gb to play with. Size varies from around 180Mb to 4.4Gb, and none of them took anywhere near 20 minutes to install. Not sure why DMC is so bad...I believe it uses ~5Gb.
If you do start to run out of space, delete the game utility data (it's held separately from the saved game data and will be reinstalled next time you put the disk in), or replace the hard drive. I've got my eye on
this atm. Swapping the drive is a piece of piss, there's a guide
here.
Unusually for Sony, who practically invented the expensive proprietary peripheral, they've been incredibly open with the PS3. You can connect a whole bunch of stuff to it, keyboards, mice, cameras, headsets, printers, external hard drives, memory sticks, etc. from pretty much any manufacturer..no having to pay the Sony tax. So expandibility isn't really a concern at the moment.