Timing depends on the ambient temp and size of the heater.

When temps here drop into the -20 to -40 (f) range, I plug in the 190D (MB block heater) when I pull into the [unheated] garage; it is always plugged in unless it is on the road. If the temps are moderately below zero, 2-3 hours is enough for an easy "summer like" start. Zero to +30, 1-2 hours seems to be plenty. Takes a bit of experimentation.

Do a search here on block heaters for more info; there are many advantages to utilizing them to the fullest extent practical. Starting warm will save substantial wear & tear on the engine.

Good luck,
Jim

__________________
2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)