When it's below about 10 degrees and I have to drive early in the morning, I'll typically start the TD when I get up and let it idle for about 45 minutes before driving it. It melts the ice of the windows and warms up the interior. I can't recall what the temp gauge reads after that amount of idling but the temperature is high enough to get the interior toasty. That's probably only a dozen days a year so I'm not about change my behavior and start scraping icy windows instead.
By the way, it is illegal to do this in Denver. It's called 'puffing'. I use two keys and always lock the car when it's running.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13