My first two questions would be how big is the engine and how many watts is the heateryour question requires several more questions before a reasonable answer can posted
1st is tractor in a garage or out in the open
2nd is tractor surrounded by sheets of plywood to block the wind
3rd is tractor covered with a quilt or similar covering
4th tractor temperature when parked
5th ambient temperature of surrounding air
6th any wind in the area
7th bare tractor or covered in ice/snow
8th power rating of the 'block heater'
9th antifreeze/water ratio
10 there's a lot more,some subtle but important.
generally speaking, if your tractor is in an unheated but draft free garage,1-4 hours before using the tractor should be OK. There's a lot of 'math' involved after you get 'real world data' to get the exact amount of time needed for optimum result, ie warm engine without wasting electrons.
I used to remotely 'setback thermostats' in small to medium sized companies3 decades ago. it took some time, but I could 'tweak' the units to get clients 35% fuel cost savings.
Sounds good. In the end....you'll find out what works for you.Thank you everybody for responding... I'm going to go with 1 hour, feeling the hose from the radiator and go on the safe side... it's an old tractor and don't want any more repair bills...
Interesting,., that's what the repair tech also told me make sure the fluid level is at the top of the radiator, and he showed me how to squeeze the outlet from the radiator to test the heat.....Sounds good. In the end....you'll find out what works for you.
Your title suggests you are concerned with 'burning out' the heater element, but the thread ultimately addressed the most 'efficient' run time.
With respect to longevity of the heater: It is a Direct Resistance Element (no different than in your typical water heater), so it will eventually wear out. Of most importance is that it is ALWAYS submerged in coolant. Otherwise it 'can' burn out in minutes.
As has already been discussed there is a threshold of run time (mostly temperature dependent) that will provide adequate heat to help start the engine vs. unnecessary waste of running full time.
Conditions (temp, wind, etc) and the possible inconvenience of you 'tending' the run time will dictate what each person chooses to do. But I wouldn't worry about burning the element out....provided it is NEVER dry (when running).