Something you should definitely add to your repertoire of quirky sayings, but as long as you use it in the same snippy context as Eric Cartman of the show South Park. Also the only acceptable phrase containing the word "hella".
Person 1: Why are you wearing sunglasses inside?
Person 2: 'Cause I'm hella cool, that's why.
Could be used to say, It's really "happening", which in this case means it's really cool, it's off the hook, it's a lot of fun...just like hella cool would imply that it's really cool, hella happening could be used in a simillar context to mean pretty much the same thing...
hella long or long hella. basically is a nor calian's way of saying Forever.
Person 1: "Dawg, I haven't seen you in hella long!"
Person 2: "I know it's been long hella!"
Person 1: "I know, dawg"
Person 2: "So Long, that i don't think anybody say's dawg anymore, man..."