Re: Inline curculation heater
It works just like a block heater (same heating element), it has an internal thermostat that will shut it off (just like a block heater) when it get to the proper temp.
As with any block heater it's best to either have it connected to a timer or plug it in an hour or so before needed.
What he said, except I would add that the time required depends a lot on the wattage of the heater compared to the engine size, the ambient temperature, and the wind exposure. A small engine with a large 800 or 1000 watt heater might get the coolant in the block mostly warmed up in an hour when it's -15°C, but that heat still needs to "soak" before the oil in the pan starts thinning. A larger engine, outside in -30° or -40°C temps, exposed to the wind may even require a second, magnetic, heater for the oil pan but even a tarp draped over the vehicle can help keep the wind out.
Most test results I've seen came up with numbers between 2 and 4 hours. I split the difference with my vehicles and set the timer for 3 hours before I plan to start the vehicle any night the temperature forecast is less than -15°C. Typical overnight lows around here are -25° to -35°C in mid-winter but do dip below -40° a few times a year.
I also have heated battery blankets installed. They make a big difference too, especially with an older battery. Block heater and blanket get plugged into a very short extension cord under the hood so I only have one plug hanging out front.