During rainy weather how many seconds should you drive behind the car in front of you?

It depends on how much traffic there is, how bad the rain is, and how fast you're driving. Definitely stay further behind the car in front of you if it's very heavy rain or if you're on the highway, but it's also safer to stay at least several car lengths (3-5 seconds) behind even in light rain or in slow traffic, because roads can always be slippery.