The light comes on at 2 gallons meaning that you have, in most cases, a safe 40 miles before you run dry. I've read and see a lot of debate in the late 90's and early 2000's model Subarus for having horribly inaccurate gas gauges that do odd things, i.e. watching the gas light turn off after taking a corner hard-only to have it come back on a few seconds later.
The gauge seems to be inaccurate for a visual reference but the trip odometer and low gas light have always been solid references in the Subaru's that I've owned. Depending on the level of care you give your car it could get any where from 275 miles to 325 on a FULL tank.