"About four or five months for typical summer contracts (October thru February) in McMurdo, three and a half months for a summer south pole contract, or thirteen months for winter-over contracts. A winter-over usually starts at the beginning of summer (October-ish) and lasts until the beginning of the next summer."

http://www.60south.com/about/faq.htm

I also really enjoyed this quote from the same page:

"The first time you come down for the adventure. The second time for the money. And the third time because you can't function anywhere else anymore."