I would go with the following formulaGoing to have to make the formula more complicated to fix that, and it highlights some of the difficulties of comparing players from different eras.
Effective longevity of a player = (Number of matches he played / Number of matches his team played) x Career Span of that player
According to my formula,
Effective longevity of Sobers = ( 93/100 ) x 20 = 18.60
Effective longevity of Kallis = (166/175) x 18 = 17.07