The rule of thumb on these flex shafts is to get into the habit of checking it frequently...at least every thirty minutes or so. Establish a baseline for what the normal or average temperature is for your flexshaft. If you notice that it is running hotter than what it normally does, then it is time to lube. It is essential that you use the correct type of lube (any liquid lube with a Moly additive). Also, you want to be sure to lube it correctly. You dont want to install and run a core that is soaked or still dripping with lube. At the factory they soak them in the Moly lube and then actually let them air dry for sometimes a day or two before installing them. You can buy a small botlle of the recommended lube from Carvewright for $5.00 a bottle if you are unable to find a Moly lube in your hardware stores.