I am up to 1.7G on a 2.0G partition.
This is assuming you are only using it for you os.
Remember that some programs will not ask you where to install and before you know it it has dumped its programs on your root drive. If you have a little more room, it will handle it and you can remove it later.
Also some applications you may want to reside on the os partition.
That is why mine has grown from 1.5 to 1.7 so far.