Depends on the weather. If your tractor is in good shape with adequate compression, it can get below 40 before you need it. Then it's like hold for 30 sec and if she lights off after 30 sec of cranking, fine. If not, go back to thermostart for another 30 sec and on (start position) till she lights off.
Then the next time you go to start at that temp, hold in for that length of time; like 1.5 min if that was what it took. And the colder, the more it takes.
You can tell if you have thermostart by looking at the intake manifold just behind where the air inlet is; on the engine side (back side) of the manifold. It looks like a humongous spark plug screwed into the manifold and has an oil line connected to it and a wire.
The wire heats up a coil and energizes a relief valve that dribbles diesel (from the oil line ) onto a red hot coil.....making as Jerry Lee Lewis used to sing "great balls of fire".
These burning blobs of diesel enter the combustion chamber and walla, you are off and running.
Now there are tractors without it and your previous owner may have had to replace the ignition switch and may have purchased the cheaper one that doesn't have the thermostart position (as stated the spring loaded one between ON and START where the dash lights; that were on in ON, go out).