Haste
Haste increases attack speed and energy generation. At level 90, ~425 haste is needed to increase your haste by 1%.
How Much Haste Should I Have?
This common question is fairly tricky to answer.
Long story short:
Enough so that you aren't energy capping, but you're still using Energizing Brew.
So what's the long story?
First, look at your trinkets. Which trinkets are you using? Are either of them Assurance of Consequence?
If yes, you probably only need around 8-9K haste, depending on your lag. Wow, that's not very much! However, that's because this trinket means that you are going to be using more Fists of Fury and more Energizing Brew. If you can't do that (and most people can't, don't feel bad), then you should probably swap to a different trinket.
If not, then you want as much haste as you can get your greedy little hands on. Many monks are going as high as 13k-15k haste. The reason isn't even just energy regen, but also because our RPPM cloak, meta, and trinkets all scale with haste. So, more haste = good! (Plus, it also lowers the channel time of Fists of Fury).