Cooling time?

I always just wait to it gets cool enough to touch without getting burned.

You're actually supposed to wait until cool, so the threads in the hole don't try to come out with the sparkplug and damage the head. This can happen sometimes if the engine is too hot. I usually just wait until it's cool enough that I don't get burned and haven't had any problems.

The reason this can happen is because of the different cooling rates of aluminum and steel.