I have no idea -- The faster you check-in, the closer to the top you are on the battlefield (in your elite group).

I generally check-in @ the 24 hour mark and have been upgraded via Battlefield probably 4 out of 12 times (as a silver). That's now changed as I'm a gold. Thats all route dependent too. IAH is a hub though (kinda hard to get an upgrade out of a hub as a silver).