I guess it would depend on your contract, if you have one.
I've only given a weeks notice - and thats me being polite. My current yard doesn't deserve any, IMO, as they're kicking us all out
I've just moved yards and gave one month's notice as a gesture of goodwill (no contract), but left before the month was up. I'd paid for the full month, but decided to forego a week's livery. It was worth it!