The pit crew decide for you. Usually 2 laps after the rain starts. Great to see a new member in this forum, but maybe dont post too many new threads for small issues. 👍
Usually, I'm leading. It all depends on how many laps are left. I won't bother if it's 5 or less. However, if I do need wets, I'll turn on the map and watch for the second place driver to pit. Then I'll send in a pit request for the lap I'm on.
I think the rain feature looks awesome in F1 CE, but unreal. If you take the wind factor under consideration, the rain must run-off your visor much faster that what the game shows. I heard that F1 drivers clear the water off their visors by shaking their heads as they do when they look to their rear-view mirrors. The wind hitting the visor plus the shaking provide temporary relieve from the water accumulating on their visors.