I have a question regarding the legality of a clause in a rental agreement. My fiancée is presently renting an apartment on a month to month basis and is planning to end the rental agreement. She has given her 30 days notice. There is a clause in the agreement stating that if the agreement is terminated prior to six months time, the security deposit is forfeit. This seems like it violates the substance of a month to month rental agreement, so what I was wondering is if it is legal for the landlord to keep her deposit in this situation (cancelling at the end of the 3rd month)?

The substance of the "month to month rental agreement" is, like any agreement, exactly what it says. Apparently the agreement is what it is.

You are confusing such agreement with a true tenancy-at-will; i.e.: an unwritten agreement or a hold-over after expiration of a lease for a term.

Unless there is a specific statute prohibiting such an agreement, and I'm unaware that there is, your fiancée is stuck.