Technically you dont need to give notice at all if you dont have a contract, but like the previous answer said, if you are owed any pay, you are unlikely to get it if you dont give a proper amount of notice- minimum of a week, up to a month, depending on how you are paid (monthly or weekly).
You should also bear in mind that if you are looking for something else, they will at some point probably ask for a reference. If you totaly shaft your ex employer by walking out without noitce, they are unlikely to give you a good reference as responsible, reliable employee.
As cr*p as the may be, for your own benefit, it is worth doing things properly to avoid giving them any grounds for making it difficult for you.
Good Luck!