During the first 4 seasons, there could have been up to 12 Fast Forwards in any one Race; when the Race reached a point where all of the remaining teams had won a fast forward, then the FF was no longer made available. I believe from memory without looking it up that the latest team that won a FF in those four seasons was Tara and Wil in season 2 on the last possible leg (assuring they made final three) but don't hold me to that.
After that, the use and benefit of the FF changed drastically and reduced the strategic value of going for a FF on any leg when it was available. Usually, the leading team at the time would go for and take the FF assuring them of a leg win, but there was little strategy involved.