Both are good examples of how people speak to each other, but they are somewhat casual. For writing, I would use the following: How long has it been since you heard from her?

Are they 'correct'? Not really, because if you analyse them grammatically, they don't make a lot of sense, because there is no real subject or verb. As @imk says, they're a reduced form of 'How long has it been since you heard from her?' However, forms such as this are perfectly acceptable in spoken language. So feel free to use them!