How long would you guess it would take a block heater to............warm a diesel motor to starting temperature, using maybe just a little ether, in say 10 degree weather?

Assume the block heater is a "circulating one" of "usual" size---- whatever that might be.

4 hours?   5 hours?

Shorter?  Longer?

I realize there are a lot of variables---- windchill, etc etc.......just looking for a "reasonable" time.