I don't see the point of waiting. I fab the reserves dry at the first opportunity. For one thing, you may need to leave the floor in a hurry, and for another, I'm not convinced that trying to remain undetected by the fab network is worth it. I fabbed four flight props in Armory and it shut down the network. If you're going to get caught with 2000+ matter available, you're going to get caught on most normal floors too. May as well empty the bank and if it fills back up enough to use again, well great. It's not as though leaving it nearly empty guarantees a lockdown anyway.