I'm at CHS so I can't speak for the other bases, but there's plenty of dudes that come in from FAIP/OSA assignments and get into airdrop. The window you hear people refer to is about the return on investment that your squadron gets from you after sending you through all the upgrades. For a guy straight out of UPT, there is plenty of time to get upgrades done and be a fully qualified airdropper for several years. As a second assignment guy, there's a tendency for commanders to be more concerned about positioning you for your O-4 board (exec, flt/cc, IP upgrade, deploy, etc). I don't agree with the system, but those boxes matter for promotion/school and being airdrop qualified doesn't. It will also depend on the PFT for the year and how many training slots your squadron gets. Some years there's enough that almost everyone can go if they want to...other years it's pretty lean. If you want to get into the program, let your DO know when you get there.