You should try shooting storms when they're far quite far from you or whey they're passing by, so you can see the structure but you don't risk damaging your camera because the precipitation core isn't directly above you ;)

According to long exposure - the last 2 photos in this post are the long exposures - the first one is about 8s and the last one is about 25-30s. Using long exposure is definitely the best way to shoot lightnings, but it works the best in the nighttime hours or during the evening. During the day it's usually to bright to make any longer exposures. You can try to use ND filters for making long exposure during a daytime, but it doesn't work good for capturing lightning strikes. The best way to capture them during a day is to buy a lighting trigger (costs about 300$) :)