Edit: okay, so the following is when airdrops are working. Apparently they are broken in A21.1 experimental. They do work in A21.0 stable.
They drop at noon at the frequency you set. They won't drop in day one if you have them set to every day, so I'm not positive of they drop on day 3 or 4 when set to every 3 days. I've never looked at the day to find out. But I am guessing they start on day 4 when set to every 3 days.
Double check that the option to display the air drops on your map is enabled. If disabled, you might just not notice. They'll still be there if you happen to drive near them, though.
I haven't had any problems with air drops myself.