On a stocked tank larger than 40gals I would agree, but with something as small as a 5gal I would wait longer just in case. The 5gal has a pretty fragile ecosystem.You could probably get away with 3-4 weeks. I have seeded filters in as little as 2 weeks but I feel that is a pretty hasty change. If you want to keep it safe, go with a month.
If you have a smaller tank with less bio-load, you have a higher chance of losing any beneficial bacteria that you have. That is my thoughts on it, honestly there is no official proof of level of bacteria in a tank, and I have no scientific proof if I am right and you could be right. But when I am taking out the sole balance of a aquarium, I am going to err on the side of caution instead of rushing by taking out a filter and risking a crash in my tank.Why would you need a 40 gallon tank to seed a filter quickly? There is less BB overall in a 5 gallon tank since the bio-load is lower. I've done the same thing with seeding my Spec V's built in filter with a sponge filter that was on another tank. It was seeded in just a few weeks. When I removed the sponge filter, there was not a detectable trace of ammonia in the aquarium.
Maybe my experience is different from yours, and I got lucky. I just think that two months is pretty long for seeding a filter in any size aquarium. Although I will say it is better to err on the side of caution, instead of rushing things.