People are speculating around 6000+ haste, crit will become more valuable. It is speculated at this point you will be energy capping a lot. I believe the actual softcap is around 6700ish (correct me if i'm wrong). I haven't seen any real napkin math on this though.
It's actually a bit more complicated than that. As haste values increase, mastery values likely will as well, which means that there will be even more mastery procs to flood your activity time with, which will further boost the value of crit relative to haste.
So generally, there isn't a number that could be labeled as a cap for haste. There will be inflection points for every WW monk at which crit becomes more valuable than haste, but those will be dependent upon both haste and mastery levels.
This is a problem, as it effectively means that of our five secondary stats (hit, exp, haste, crit, mastery), all but crit will hit a soft or hard cap within the first tier of heroic raiding.
Hit will be soft-capped, and will continue to boost autoattack damage for DW setups.
Exp will be soft-capped for attacking from behind, which might as well be a hard cap.
Haste will be soft-capped by virtue of ability flooding and energy capping.
Crit won't be capped yet.
Mastery, while not great, gets even worse when we become GCD capped, as the Tiger-palm half of it won't even be used for filler--only used to keep up the buff.
If our mastery made its procs not trigger the GCD, that could do a lot for improving our gearing situation in this expansion.