There’s not really a strict guideline for how long you should invert, as the time largely depends on you, your specific condition, and whether you have already had some experience with inversion tables or not. However, there is a general rule of thumb that can apply if we roughly divide users into beginners and intermediates. Any pro-level users out there are probably professional athletes, which should know the ropes by now.
To answer the question, beginners would do well to invert 1 or 2 minutes per session, or as far as you feel comfortable. This is at the very beginning, and you can extend it to 5 minutes once you get more comfortable and confident. It’s not unusual for beginners to invert for 5 to 15 minutes down the road, but you should stop it the second you start feeling discomfort. Bottom line is, don’t do it too much and/or too fast. As a matter of fact, the more important question here would be how often rather than how much. In general, at least two times a day is the minimum, once before breakfast, and again just before going to bed. For maximum results, you should probably do it one to two more times during the day, at your leisure.
As far as intermediate(or experienced) users go, it’s perfectly conceivable to do it up to half an hour, but not really recommended. Who qualifies for the title? To put it simply, anyone who has used an inversion table at least several times over the course of a couple of weeks or so. In other words – everyone who knows their body and their limits, and is not liable to overdo it, qualifies. So, just like the beginners, experienced users should not exceed 15 minutes per session, though they can get away with half a dozen sessions a day. Conversely, beginners could and should make do with two sessions a day, at least in the very beginning. Long story short – see what feels comfortable for you, and stick to it.
Of course, the question is not simply one of duration and frequency, but also degree (in a more literal sense than you might imagine). A rule of thumb says that the steeper the angle, the shorter the time, and vice versa – the shallower the angle, the longer the time you will be able to invert.
Both periods will increase progressively the more experience you accrue. Of course, there’s one final thing to note here – all of the discomforts you may feel when inverting will stem from going too far, too fast. In other words, the human body is a marvel – it is well enough designed to be upside down, risk-free, but if you’re anything like an average person, you and you’re your body is not trained to be like that. Long story short –start slow, and increase the angle and time steadily, as you become more and more comfortable.