If you have your feeds and speeds correct, then the HF-500 should run for a very long time without overheating.  There is an internal air cooling system build into the unit that will keep it running at optimal temperatures. But If you over stress the spindle, you can cause it to run hotter.  The key is to ensure that you are not moving the bit through the material too fast and that you are not forcing the bit/spindle to remove too much material per pass. If the spindle is too hot to touch, then you are doing something wrong with your feed rate or depth per pass. You should always be able to touch the spindle and it should feel warm.  If you can’t keep your hand on it for more than 15 seconds, then you need to look at your setup to ensure that you do not cause damage to the spindle.