It varies as XP only saves anything changed during System
Restore, and maximum HDD space allocated is 12%. To check
yours, right click on My Computer and left click on
Properties. Now click on the System Restore tab. Click on
your drive and then Settings, you can see then how much
space is allocated.
My PC is currently holding 10 restore points and the
largest one is 55MB
You will find the restore points in C:\System Volume
Information\_restore. These are hidden folders that you
do not usualy have access to though so if you have XP
Pro\Media Centre you can grant yourself access and take a
look.