In Safari items in the history are maintained for 7 days. While other browsers like Firefox let you set this value as a preference, Safari has no obvious way to change this value.

But of course just cause it's not obvious doesn't mean it can't be done :)

To change the history lifetime in Safari, open a terminal window put in this command:

defaults write com.apple.Safari HistoryAgeInDaysLimit 30

The number on the end, 30 in the example, is the number of days that Safari will maintain a site in the history. Set this value to whatever you want.