usually it is at least a 30 day cycle which means that you owe money or rent for the entire cycle, assuming it is 30 days, it so ends.
You are entitled to a walk through and a list of any items which the landlord may think of deducting. You are entitled to know which account, interest bearing, the money was placed, and the amount of interest.
The rest is purely math.
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How much was the monthly rent? In MA, a landlord can only charge up to one month's rent for security. If he or she did not handle the security deposit correctly, you may be able to a full refund (landlord is estopped from damages in certain cases). Regardless, you should send a 30 day demand for the return of whatever amount you are entitled. A good housing attorney may be able to get you up to 3 times what you paid plus attorney fees.
The two other lawyers are correct about the issues they pointed out. The Security Deposit law in Mass is very technical and specific about the handling of the funds and it trips up a lot of landlords. There are also potentially harsh penalties (i.e. triple the amount of the deposit) if there is a violation. You should consult with a landlord/tenant attorney if you believe there is a violation.