Not sure how long if the AC20 is just sitting there with just the original BioMax and sponge it came with. But if you seed the filter compartment with media from the other filter that has been running (assuming its been established and running for quite some time) then it wouldn't have to sit there long at all.
I actually seeded my AC20 by placing that BioMax bag in my other (much larger) filter that has been running for about a year. I left it there for 2 weeks and I even squeezed some filter sponge brownish BB juice on it for good measure. Afterwards, I placed the BioMax bag in the AC20 and then on my Fluval Edge tank and proceeded to stock a little bit at a time with no problems. My Edge has been running for 2 months now, everything checks out perfectly on my test kit params.