If you are shooting lightning, the exposures can be a long as you want. I like to sit with a tripod and cable release. Simply holding down the button take the picture...I count to 10 and if there isn't any lightning I start over. As soon as the lightning strikes, your Aperture and ISO will determine how blinding-bright the lightning will be. Simply release the shutter after the strike.
I start a new frame after 10 seconds or so to combat long-exposure interference. If you have hot pixels, they will rear their ugly head in a 15+second exposure!