Timed on/off switch problem
Here are some commercial solution that seems to do what you want:


intermatic.com/.../ei400_series
quasarelectronics.co.uk/.../Velleman K8075


To build it yourself the most popular solution is a 555 timer, may circuit schematics are available online. Add an appropriate bridge rectifier to replace the VCC/Battery input and a relay at the output for AC operation. This link is the opposite of your application, but the "edits" illustrate how to add the rectifier and relay:


electrolund.com/2007/10/ac-outlet-timer-circuit/


This link uses a 7555 with a range of resistance values for adjustable timing:


www.zen22142.zen.co.uk/Circuits/Timing/5_30timer.htm


Have Fun!