What is the maximum number of days when travelling in which one can combine prayers?

There are many different opinions on this subject, but as we know, it is permissble to combine, and shorten prayers when travelling (on vacation for example). As such, one would be allowed to pray dhur and asr together, with 2 raqaat each for example. But when is the maximum number of days in which one can do this? I've heard that if you are in a certain place for more than 3 days, then you are not permitted to combine and shorten prayers. Others says 15 days, and some say as long as you are far from your home, however long that may be, you are permitted to shorten and combine prayers.