However long you decide please keep notes and let us know what is actually a safe amount of time. It would be pretty easy to keep the sponge in an established tank for 3 days then move it to an empty tank and dose ammonia to say 3ppm, if you can't measure any ammonia or nitrite after 24 hours I'd say the filter is seeded, if you have ammonia or nitrite after 24 hours put the filter back in the established tank for a few more days and repeat the test in the control tank. I have a feeling it wouldn't take much more than 4-5 days to seed a sponge filter but I've never had filters or tanks to spare to perform the experiment.
3ppm is a pretty hefty amount of ammonia. Even a fully seeded filter that was left in the tank for 4-6 weeks may not handle that in 24 hours. It gets complicated because you have to know the ammonia output of the fish involved. And then every tank is different, so you'll never come up with an answer for everyone. Best that can be determined is a range and that'd be anything from about 1 to 4 weeks, I'd guess.
Certainly would still be interesting to hear your results though. :thumb:
I might try doing it tommorow at the 4 day mark. I dont know if it makes any difference but there is a heavy bio load in the tank that is seeding it and the bio load will be very small in the new tank. And the filter is for up to 25 gallons and I am putting it in a 10 gallon.