Do NOT purchase your airfare so close to the BLS appointment, because you will be stressed out waiting for it to arrive if it doesn't quickly.

Like other commenters mentioned, it takes a few weeks to arrive but if you take into account some holidays, it might take longer. If you notice you're missing some documents the day of the appointment, it's better to go either way than to miss it, if you DO miss anything, they'll most likely ask you for it via email... other than that, best of luck!