again, for such a simple problem with a simple solution, you all have to extreme complexities like PIC and temperature sensors and magnetic sensors and all that crap. there is no need to go that for just to have a simple piezo buzzer sound when the door is open too long. its like constructing a Rube Goldberg contraption just to open a door when the door handle is pulled. by just putting a small 555 timer circuit that pulses to trigger a transistor leading to a transistor oscillator circuit that will drive the piezo buzzer, if you assemble the circuit correctly you can get an alarm that buzzes when you leave your freezer, or any door for that matter, open for more than the time you give for the circuit.
estimated cost: $3.00 to $9.00 using parts from Radioshack. using surplus stores will drive the price down even more.
Just think about it, even though its too simple for you to want to consider, and then think about how much it would cost to get temperature sensors, PIC chips, and complex control circuits just to make an alarm buzz because the fridge was open too long.