Is there a rule of thumb for how many wires you need to take up a gate’s worth of space? I'm trying to figure out how much gate reuse makes sense. Although if ASICs aren't at that low a level I'll stop worrying about it.

Not sure what the question is about. Depends on the size of the gate. Take a look at the layout and compare it to minimum step metal grid and see. What does "asic are not that low level" mean?