Depends upon the situation.
If you need to get the hell out, then at least equal to what you're making.
If you're in a good place and someone offers you, then you want no less than a 10% increase and typically 20% to make sure that you don't use it to negotiate for more at your present job.
FWIW, one thing to keep in mind, you will typically have one opportunity at most to tell your present employer that you've been offered a job and they'll negotiate to keep you. After that, you're marked and it will be a situation to be aware of in the future.
Lots of options, but like fan4life said, it really comes down to what feels right.