Like a couple have said, it depends on the situation.
Panfishing, its alright, because there are huge numbers of fish, that are quite easy to catch, so its not a big deal.
Trolling...about a cast length is acceptable, but give as much space as possible.
Casting....well, thats totally up to you, but for a ballpark you cant be within 150 feet of another boat during a tournament.
I tend to stick with that as a guideline.
Treat others as you wish to be treated.
(If they dont return the favour, tie on a few egg sinkers!)