How Much is Too Much? (Diminishing Returns)

Really need some help with Stat balancing, specifically Haste and Crit.

I'm not really the type of player that keeps around several sets of gear to test out the stats work, I'm more the guy that works with the knowledge he's given. And right now there isn't much out there regarding how much of a stat is too much.

With that said, anyone know when Diminish Returns start for either of these two stats? And is Determination even worth aiming for, or is it just nice to have?