How to set the exposure time to 15 mins on a nikon d40?

How do i set the exposure time to 15 mins on a nikon d40?

You set the cameras shutter speed to "bulb"
You use a remote shutter release to lock the shutter open
You use a stopwatch to time your exposure.

This applies to ANY fully adjustable camera, not just the D40

Besides using a manual release and a stopwatch as Ace suggested, you should be able to find one for the D40 that has a timer, like… Http://www.bhphotovideo.com/c/product/407310-REG/Nikon_4917_MC_36_Multi_Function_Remote.html

Turn the mode dial to S and then spin the thumb wheel on the upper right. In the viewfinder and on the display screen, you will see the shutter speed change. Turning it to the left will lengthen shutter speed, right shorten it. You will be able to set it up to 30 minutes. If you want bulb, meaning you have to hold the shutter button down to keep the shutter open, use the M setting and it will go from 30 minutes to "bulb". If you have the $20 infrared remote shutter control, you can simply set it on bulb, click once to open and click again to close it.