Controlling an electromagnetic switch with Reed switches
You could use a large capacitor in series with the reed switches and the magnet. When the reed switches close, current will flow though the capacitor (and thus the magnet) at first, but as the capacitor charges it will conduct less and less current. You should add a resistor across the capacitor leads so it can discharge when the switches open. 
Use a 555 timer circuit in one-shot mode. 
Get a commercial timer board designed for exactly this purpose.