It will not fire if temperatures don't call for it.
That depends on the heater and what range snap switch it has specced. Some are for warmup and some are for maintenance. The gensets I maintain have a switch that keeps them cycling occasionally and the engine around 70-80 degrees.
As for use:
How long you run it depends on the temp and how big(in watts) the heater is. Personally I prefer the external tank type heaters as due to available space limitations inside an engine block you can be limited in how large an element you can get.
The tank type heaters can have larger elements(1000+ watts) which means shorter warmup times. A 15A residential circuit should support up to 1800 watts, neyond that you may need a special circuit and dedicated plugin but 2-3KW are not unheard of. I personally think they are a little easier to install. Because they thermosiphon/circulate the coolant, they deliver the heat directly to the head via a heater supply port and draw water from a block drain or waterpump return port so have a little faster warmup IMO.
Some just plugin when needed. That is what I do on my tractor, I have a 1KW tank type and I go plug it in 15-20 minutes before I try and start the tractor, nice and warm and delivers good clean startups. Some run them on timers to preheat before normal use times, like 1/2 hour befor you normally leave for work. I do this with a space heater in the winter on my gas powered truck, havnt scraped a window in years...
If you have a smaller element, you may need to leave it plugged in to try and keep it at a reasonable temp as it might take hours to take the edge off...