Used much in the same context that others not from norcal use the word forever (eg) "We've been living here forever"
norcal person 2: Yeah dude, I know. The traffic in the bay area blows.
norcal person 1: I haven't seen you in hella days! (usually 3 or more days... but it could even be years)
norcal person 2: hella
it can also be a sign of agreament
guy 2: ya i know its been awhile
guy 1: dude this is some hard shit
guy 2: hella dayssss