Hi....i presume you are adding this filter to an already established trop tank...possibly in order to replace the old filter (which will be removed ?)
If so, the longer you leave the new filter running, the better. At least a week, preferably two would be my choice.
When you remove your old filter, just keep an eye on your feeding. Maybe feed every other day for a week (it won't hurt the fish) just to stop any overload of ammonia from the fish. Don't add any new fish either for a few weeks.
Good luck