On average, boxing careers of men in the present day last about five years. Before 1995, the average was almost twice that. Part of what accounts for this change is the increasing intensity and rigor of boxing as a professional sport. Women boxers, however, do not have any documented career span, due to the fact that women’s boxing was only recognized as a professional sport a few decades ago. Notable women boxers have had very different career lengths, based on their varying interests and other commitments.
 
Christy Martin (b.1968), an American-born woman boxer, began her career in 1989 and is still training. Her career length, currently spanning over a decade, is not unheard among women boxers. Marian “Lady Tyger Trimiar” (b.1953) boxed professionally for 14 years (from 1973 until 1987). And, of course, Fortnight luminary mentor Lucia Rijker (b. 1967) stepped into the ring in 1988, took a brief three-year hiatus, and then boxed until 2004 (though she is not officially retired). On the other hand, some careers have been shorter. Jacqui Frazier (b. 1962), daughter of the legendary Joe Frazier, boxed from 2000 until 2004, when she retired from boxing to become a judge.
 
Sources:

http://www.luciarijker.net/
http://www.womensconference.org/laila-ali/
http://www.womenboxing.com/tyger.htm
http://www.eastsideboxing.com/news.php?p=13753&more=1
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1725298/pdf/v039p00661.pdf