Hmmmm...
It
looks like you can set it using the Terminal. Open your Terminal (in /Applications/Utilities folder) then type:
Code:
defaults write com.apple.safari HistoryAgeInDaysLimit 14
...and hit return. Change the number on the end to whatever number of days you like.
I say it
looks like you can do it that way because I'd have to wait 15 days to be certain! ;-)
Good luck