The answer varies with different people. Beginners should start at a mild angle (approximately 20-30 degrees beyond horizontal) for the first few weeks until you become comfortable with the operation of the table and are able to completely relax while inverted.
20 - 30 degrees: At this angle, your body begins to experience mild stretching to your muscles and joints, while benefiting from stimulated circulation, improved oxygen flow to the head, and repositioning of internal organs.
50/60 degrees (parallel with the rear legs of the table): This is the angle to which the average person experiences virtually all the benefits of inversion. Your spine receives the amount of traction it needs to completely decompress (once you are relaxed). Most people don't really need to go beyond this angle.
Intermittent traction / Oscillation Intermittent traction (alternating 20-30 seconds inversion with returning upright) or oscillation (rhythmic rocking back and forth) are actually the "preferred" methods of inversion, recommended by many doctors, for stimulating circulation and waste removal in and around injured discs.
Virtually all issues of discomfort that occur with new inverters are due to going too far, too fast. You are wonderfully designed to be upside down, but if you are like most people, you are detrained to be inverted. Just listen to your body, increasing your angle of inversion only as you feel comfortable.