The largest apps I've seen are like 70 megabytes (there's probably some that are larger, but I haven't seen them yet) and I believe you only get 9 pages of applications. meaning that even if you installed nothing but 70 megabyte average size apps (most that I've installed are less than 5 megabytes) then you can't go over 9 GB of applications on the touch at any given time.
It might be different if you jailbreak it, I'm not familiar with doing that. But if it's a 2G then jailbreaking isn't really that practical right now anyway.
If you have a 32 GB touch, you really can't expect to have less than 20 GB of room for music and video after you fill it with apps.
But you've gotta account for Safari, Mail, Calendar, Contacts, YouTube, Stocks, Maps, Weather, Clock, Calculator, Settings, and App Store. The basic ones that come pre-installed and AFAIK can't be removed.