How hot should the oil be when preparing schnitzel?
I generally use about 1cm deep of olive oil when I'm making schnitzel which may or may not be the right oil but it works for me.

I generally find that if the oil is smoking, it's too hot so I tend to get it to a temperature that is very hot, but not smoking.

Once I get it to that temperature, I don't cook too many schnitzel's at once, generally 2 or maybe 3 max to a pan depending on how big your pan is.

The final thing is that you need to make sure you beat the schnitzel out nice and thin before you crumb it so it's only about 1/2 a cm thick when you're cooking it. This way it'll only take a minute or two to cook through and won't overcook the crumb.