If it's one or two fish, I'd say rightaway. Just monitor the
waterquality very securely and perhaps add some bacteria-starters to
the filter. But depending on the medicin (if any) you're going to use,
you might kill off the bacteria anyway. Small daily or bi-daily
waterchanges and waste removal from the bottom are more important.
If it's a bunch of fish, I'd consider it like a regular tank. So a
startup period of 2 or 3 weeks.
That's what I would do. :)
Gunther