Search the forum, you'll find loads of info.
I fill up when I get to the last quarter.
At this point I usually have around 310 miles covered, and the computer tells me I can go on for another 80 miles or so.
This is a typical week to work and back though - mainly motorway and duel carriageway.
In town is a different story - and putting your foot down is an even more different story!
Really depends on how you drive and the road conditions, but the GTI is pretty economical compared to other cars in its class.