Pre-heating

Our FBO (who are otherwise OK people) have a new policy. They will not
allow one to leave one's airplane's preheater plugged in continously. Instead, one must call them ahead of time and they will plug
it in for you. They charge $10 for this
service.

My questions are this: How long does it take to pre-heat an engine? And will pre-heating my engine prevent more than $10 of damage
per flight? Finally, how do you know? It's more reassuring to here from a mechanic than some tale that starts "My grandfather's
uncle noticed ..."

My particulars are

A Cessna 150
Tannis preheater
It's currently 30 degrees outside.
About 1500 SMOH