As the others have said, it should be in your contract, and if it isn't then you should question it.
It should be a pro-rated amount compared to the full time amount.
So if you are working 20 hours a week, and your full-time colleagues are working 35 hours a week, you should get 0.57 of whatever they get.
This may be expressed as days (which will mean YOUR days, ie. four hours), or hours.
Full time employees should get a minumum of 28 days a year, which can include bank holidays.