There is no "statutory" rate, or rate set by law. The interest rate is whatever the current rate on savings accounts is. Interest rates last year were around 1% or less. Even if it was 1%, then 1% of $300 for 1 year is $3.00, and for 15 months would be $3.75. Even if the landlord did "stiff" you, the most you could sue for the additional interest you should have received--that is, another $2.75 or so.