Quite a bit. As long as the streamer is running 3 minutes of ads per hour there will not be prerolls. Ad breaks can be run manually or on a configurable schedule, and scheduled ad breaks can be snoozed for 5 minutes (up to 3 times I think) if they arrive at a bad time.
You can't stack preroll free-time for more than an hour though, so you don't get 5 hours without prerolls by running 15 minutes of ads at once.