The following is an non-stable oscillator circuit which will automatically connect and disconnect your soldering iron every 2-5 minutes or so, check it out.

You can experiment with the values, in order to control the duty cycle, frequency and pulse width. You can also try using the TLC7555 instead.

You can assemble this circuit (With its DC power supply) in a box permanently connected to the iron. For this you can also connect a switch in parallel with the relay contacts if you want the iron to stay on.