Power consumption of MOSFET
Look for the off state leakage current spec for the MOSFET you are using. Usually called Idss or zero gate voltage drain current.  Typically it's in the range of  a few uA, which isn't a lot of current draw but you would have to evaluate the leakage against the battery capacity to see if it will significantly impact the shelf life.  The leakage can increase dramatically with temperature, so if high temperature storage is a possibility be sure to take that into account.