My advice. Never. In most parts of the English-speaking world people will think you are an idiot if you say it. It would be very weird for a non-native speaker to use what is region specific slang.

In summation : It is a truncation of “hell of a” or "hell of a lot (of)" It is definitely far too regional, even in just America, to be an all purpose word. Using the full version “hell of a” can be considered vulgar by some people because “hell” is often a swear word when used alone. (hellfire and hellbent are not considered swear words). It’s also the way some people say “hello”: “helluh there felluh”

I agree with David. It is used the same way here in Texas. However, "heck" is less objectionable among people you don't know well because some of us try not to use profanity ("hell," "damn," etc.) or vulgarities ("fuck," "screw you," etc.) Instead of using those expressions, you could just use another intensive -- as in David's example, "He's a great guy."

I have never heard of "hella" but it's interesting to hear that it's slang in California. In the UK (and maybe the US?) we use "helluva" or "heckuva" as in: "He's a helluva guy!", e.g He's a great guy. Or: "That's a helluva a big capuccino you have there." It's a corruption of the phrase "hell of a" (and "heck of a") I believe.

It means "extrmely" as in: He's hella intelligent. It also means "a lot of" as in: I wanna drink hella coke.