Watching the seat maps just after the 100hr & 72hr marks often shows how many elites made the upgrade at that time.

I've noticed that just after the 100nr & 72hr marks lots of exit row seats open up.

When GA's are doing there final boarding juggle they need to find isle and exit row seats more often than middle or window seats. They're only going to give you the op-up if they need your particular seat. This is a game of chance, it depends on how many people are travelling in large groups, how many families with kids, how many non-rev FA's, pilots etc. There is no doubt though that your unlikely to get an op-up from a center seat, they rarely will need it.

I've often found that when I stand-by, I get a nice exit seat and the previous reservee gets an op-up.

I'm beginning to hate the exit rows, although you have the extra legroom and often find yourself seated nexto other elites, the seat cushions are a good 3" shorter on most exit row seats, something that translates into a a pain in the ar5e for me.