This is probably the most commonly asked question about inversion. The answer really varies with different people. For the most part, we recommend beginners to start slowly: invert for only 1-2 minutes during the first week or two until you become comfortable, and then work up to longer periods of time as you feel necessary. The angle of inversion also affects the length of inversion time that is comfortable. The shallower the angle, the longer the time. Most people will invert for 5 - 15 minutes once or twice a day. There is no real time limit-the important thing is to listen to your body. If you are at all uncomfortable, simply return upright.
Virtually all issues of discomfort that occur with new invertees is 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 the duration of inversion only as you feel comfortable.