I replaced my brake booster but I don't know how to adjust the rod between the booster and master cylinder. What determines how much or how little you unsrew it? Is it determined by pedal travel or something else?

If the new booster came with a new pushrod, you can compare the adjustment with your old rod, and adjust it accordingly.
If you're re-using your old rod, then no adjustment should be needed.
If you need to adjust the rod, then it should be adjusted to where there is maybe 1/4" or less of free-play at the pedal. I'm not sure if there is an adjustment spec on this, but I can look and see...I can't say that I've ever even thought about it...now ya got me wonderin...

Thanks Joel; It came with a new rod so I used it. I did'nt change it any and I have about a 1/4 " of play so I think I'll leave  it alone. I could not find anything in any of my books about adjusting it. You guy's said i would have fun changing the booster and you were right. It looked like it was the factory one. The nuts did not want to come off and I think I lost about a pound of skin off my knuckles and my back still hurts and I put it on last week.

I think that is one of the hardest jobs I have had to do on any car. You need to be a contortionist to get in there. Motrin is a must have before and after the job.

The 1/4 inch play is about right.  Slightly less would be okay, but keep in mind this measurement changes with temp changes.  You should be fine.