The answer depends entirely on your own personal fitness, or are you asking about our personal fitness. In my case pretty soon after I stopped as my pulse was dropping to zero...
As Bigeye74 states, it really depends on personal fitness, but I think your probably woundering what would be the average for a healthy, fit person, so you could compare yourself.. if thats the case you should visit, http://www.netfit.co.uk/ty13.htm
you can find out how fit you are just by checking your resting heart rate against the table.. hope this helps.