All of my stamina drops have been at the high end. Since they indicated that the rate of decline was based on the current level, we might hypothesize that the decline is (guessing) 10% per week. This would mean that a 9 would drop to an 8 in 11 weeks (almost a season), and a 4 to a 3 in 25 weeks (almost 2 seasons).
There are sublevels, so your player may have only been a bit above 4.