Severe thunderstorms can easily move at 45-55mph so if you see dark clouds there isn't really a way to determine how soon they'll be there. There is no rule of thumb. They can also popup and intensify (or fizzle) as they approach. One moment nothing on the radar and 30 minutes later a strong storm. Keep an eye to the sky and the other on the weather radar. Also, you can download weather radio apps on your phone if your marine radio doesn't have one.
And you definitely do not want to situate yourself next to anything that can be struck by lightning. The voltages are so high, the same rules don't apply.
http://www.boatsafe.com/nauticalknowhow/lightning.htm