I also rely on the dash mounted indicator lights getting brighter and that click to know the GP cycle is complete. On my '87 603 it seems to be about 45 seconds (I haven't timed it). It certainly is long after the GP light has gone off. I use a similar method with the '84 and that GP light goes out much sooner than the '87.

I just used the block heater for the first time and was extremely pleased with the results.

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Steve
'87 300TD - 132K - Soon 4-Sale
'84 300D Turbo - 122K - Driving
'77 VW Type II - 77K - Restored
'08 250EX Ninja
English Bulldog (Brier) - My best friend. Passed away 12/02/04 while in my arms.