With the new posting times (every 15 minutes) I'm as clueless as everyone else. There is no "every 5-6 hour" piece at play though, I can tell you that much. Both my gifs featured within 2 hours this morning. It's more to do with when the mods approve things, but they don't seem to do it at the same time everyday now.
I've also submitted two customs and a popular template at the same time. The two customs featured in just over an hour, but the popular template took 9 hours.