"how often is the lead vocal in the same room as the drums?"
I'm going to guess that it's not so much about the room as it is the plugins.
How much is right?
Take The Church and Under the Milky Way - LOTS of reverb.
https://www.youtube.com/watch?v=pWxJEIz7sSA
Other songs not so much. Some songs just enough to "blow a little air" into the mix. Unless there's a moment of semi quiet, you don't even hear the reverb unless you muted it.