It's really tough to call -- depends on how busy each place is on the night you go and the luck of the draw (which also factors in the size of your party, etc). We generally aim (when we can) to check in for ADRs early -- 10 or 15 minutes; we've even been lucky to get seated immediately a couple of times when, because we were hungry and near the place, we checked in 30 minutes early just to see. I'm sorry, but it's really not more predictable than that in my experience; if you can indeed be prepared to wait 30 minutes or so, then you'll perhaps be pleasantly surprised more times than you're put out by the wait.