Depends on the model/year.

My 87 300D will go for about 36 seconds. If it is really quiet you can hear the relay click off. Another way to check is to watch your dome light. It will brighten up when the relay kicks off.

-Jim

__________________
1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.

This is an answer in a thread about a 1980 vehicle....
but those are ways to see how long your plugs are drawing current...
The advice about leaving it on...as compared to ' glowing it twice' is still good since many people don't realize it is suggested in the FSM...