as a rule its generally 28 days written notice when vacating a rental whether its at the end of lease or a broken lease. and if u beak a lease theres generally a fee for doing so.
so ur parents would have to pay 28 days from the date written notice is given.
it sounds like the real estate is being a bit dodge, but ur parents will most likely have to pay the 28 days.
if they still dnt think its right that they should pay it, take it up with the tribunal(sp?)