My general rule is to check myself every 5 levels before 50. Dungeon gear I consider to be good for about 10 levels, whites for 5 levels. I don't sweat it too much if a slot or 2 falls behind as long as most stuff is close to current.

Post 50... 2 levels or so. But leveling takes long enough and the dungeons are pretty generous with chests that keeping up to date hasn't been an issue for me (and if you have AIW/Dreadwyrm, you don't need drops till the 57 dungeon anyways). Still, same principle. If RNGsus really doesn't like me, fill in some slots with vendor whites (or check MB as sometimes it's cheaper).

Once you're at level cap... just get the highest ilvl everything you can. Worry about offstats only once you have more than one piece to choose from at the same ilvl.