4 months

http://www.tf2outpost.com/trade/8931633

got offered 3.5, 3.8, and 4 basically said he accepted 3.8 and 4

http://www.tf2outpost.com/trade/9795032 alot of 2 pure and 2.5 all declined

http://www.tf2outpost.com/trade/6517925 got offered 3 pure 3 times, all declined

http://www.tf2outpost.com/trade/6333934 says bo is 4 buds got offered 4, 3.1, 3, 3, 3, 3.5, 3.8, and 3 adds for his bo of 4

some guy said he sold his for 3.8

http://www.tf2outpost.com/trade/10266243 - sold for 3.5

http://www.tf2outpost.com/trade/9845370 - sold for 3 buds + 23 ref

as you can see basically many offers and adds of 4, but doesnt seem to go any higher. some people say they sold theirs for 3.8 some people sell quickly for 3.5

~(>_<。)~shenanigans suggested 3.5-4