I do something similar usually stopping around 45 minutes. I've gone to this method over a couple years of trial and error and it seems to be ideal for me. I have also come across studies which show that people can only focus on something for about 40-50 minutes before losing interest. So a short break is needed after that time period to refresh.

Also I have noticed that I tend to accomplish something worthy (eg. fixing a bug, implementing a small feature) at around the 45 minute point so it is good for motivation as I can take a break knowing I have accomplished something.