How often do you update black market rewards?

Uh... If you are referring to how often new Crates get added, about every 2 Weeks.

If you are referring to the EP Mall, I'd imagine every few Months.

And if you are referring to how often the Temp Rewards in said Crates get changed, that's something quite a few others have taken issue with, as it doesn't seem to happen often, with the same few temps being in most crates.