AJ,

That's kind of a trick question because too many factors influence the answer. If the temp is 32 an 8V92 will start without the block heater, but it will be a little cranky and it will puff out white unburned fuel until all cylinders start to fire. That is an engine that has not run the previous day and is cold soaked. If you just shut that engine off 12 hours before it will likely start up almost like normal.

If it gets down to 20 and the engine is cold soaked you need the block heater. How long depends on if it is windy, and if the engine had run the previous day. I would run the block heater about 3 hours on a cold soaked engine in 20 degree temps.

If it is 10 above zero I would probably have the block heater on all night. It is safe to run all night.

If you have Webasto that will keep your engine at operating temperature without the use of a block heater. I don't know about Espar or Aquahot because I am not sure how they are plumbed.