Shadow Copy disk space (2003)

I'm planning the partitioning on a new Win 2003 implementation and need to know the sort of compression ratio you get when planning disk space requirements.

That is, If I have 20GB of files, for example, and want to keep 3 shadow copy versions at all times (last night, yesterday and last week) what sort of disk space should I allow for?

Can anyone find a link or let me know out of experience of their implementation what I should be planning for?