I don't know how to explain it better than just point to braukaiser.com. Any time I have followed his steps, I've been able to see where I'm at with the conversion & have been able to get a good idea what's happening.
http://www.braukaiser.com/wiki/index.php?title=Starch_Test
I don't disagree with the average times given, but I guess I'm thinking if you're out of the ballpark, you can let it rest for 2 hours & still not get good conversion. I'd rather test it.