Every time the in-game date reaches the 2nd of the month, a new slot opens. So not too hard to work out when the next slot will appear, to the nearest 10 minutes or so. Somwhere around Jun 55 it was late in the month and the slot was still there. Dec 55 went in under 3 minutes, Jan 55 went in under 30 seconds, Feb 55 also went in under 30 seconds. I got that Feb one, so no idea how fast the next 7 went.
You should like my airline in this world, too.