I dunno, but I understand that it's a) kinda short by Western standards and b) gotten longer by Thai standards. A number of rules changes and provisions for safety (i.e. the intoduction of timed rounds and breaks in between, the introduction of cups, restrictions on certain strikes) in the past 25-30 years or so have allowed some fighters to continue an extra couple years or so and then retire with lower incidences of permanent brain injury and mashed testicles than in years past.
EVen by the standards of other contact sports, Muay Thai exacts a heavy physical toll on the fighters, who tend to have fairly short careers. Years ago, most good competitive fighters aspired to last a few years and collect enough fight purses to buy a plot of land and retire into farming; dunno how much that's changed.