They get recharged exactly 24 hours after you did them.
In other cases, if you do 15 invites at 10 am and the remaining at 11 am, you'll get 15 recharged at 10 am the next day and the remaining at 11 am the next day.
I've seen glitches, though. I got 15 recharged for free once. :P
Hi,
What time in PST does the invites recharge? In one of my groups, the invites recharged 3 times today.
-TCN