I recommend running each players code for 1ms but do as luke says above, at the point they get a time-out, move to the next playrs code, and continue later on
I think that is pointlessly over complex. I think if you run each line of code in turn as Luke suggests you're going to be fine.
If the game core is running at say 120 updates per second I don't think it's going to make a lot of difference. If you've only got 4 players that means each player will get 30 lines of code executed every second.
Each player will have their own set of variables so running each script line in turn isn't going to be a problem.
And as Luke said, it gives people with longer scripts a minor disadvantage over the more simplistic scripts.