Not so simple this.
For each map allow for 1GB per map you create as a standard if not more.
When you compile lighting and the map you get some large files created
You have the .led and the pack files. These can easily become massive.
When I am home I can post examples of sizes.
If you have a few projects on the go you may eat a further 10-20gb up easy
You then have the fact the current tools do not have many assets atm.
If we get more assets which I hope we do, again the size will balloon further,