This is a difficult question as it depends heavily on a) how much abuse it will endure and b) the quality of servicing and maintenance.  The following is an indication.
Trailers generally have 3 cycles:
5-6 years - Primary use contract (Body structure under Don-Bur guarantee)
5-6 years - Primary continuation after mid-life refurbishment
~10 years - Secondary use
Considering the above, a trailer's primary life will last between 10 and 12 years in a reasonably good state of repair.  Thereafter, for the following 10 years or so, maintenance costs will increase significantly although the trailer may remain perfectly viable.
After 20-22 years, it is expected that the trailer will be either broken down or transported to the 3rd world for re-purposing.