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.