It varies greatly.
I would say 2/3 to 3/4 of what you could do on the track.
Some thoughts
Able to do less distance:
It does tire out your legs quicker if you are not used to hills.
You go less distance for the same effort and time (400s on the track for me vary from 55-65 seconds, on a 10-12% slope it is more like 1:15-1:25 or a little slower for the same effort. So you are actually doing more work for the same distance(20+ seconds more).
Also take into account the fact that on the track you can take a standing break after each rep or a 100-200m jog. For hill you generally have to run down, which is jarring for the body and doubles the distance.
More distance:
Less pounding going uphill. That's about all.
The one exception in short uphill sprints with a walk back. Compared to short sprints on the track, you can do a lot more on a hill without getting injured.