That depends, I've read even on this forum that it will harm the fishes if kept longer, and that it should only be used to filter-out the remaining of the medication in the tank; it's said that it should be taken out (or replaced if necessary) after a week.
...on the other hand I read, that it can be kept for long and it doesn't hurt
Although that was in vast minority of the opinions I've came to contact with 
So-- choose whatever you please
