Pon Farr as a concept is confusing. If it strikes once every 7 years, didn't Spock experience it in between his time while serving under Christopher Pike's command to Amok Time? When does the cycle start, when males finish puberty at 14?
Since Spock is half human, he didn't undergo pon farr until "Amok Time". He specifically says there that since he hasn't had any bouts of it already, he thought he would never get it - suggesting that most Vulcans do get it before their thirties.
Spock being half human screws up his "second time around", in ST3:TSfS, too. There, he is said to be suffering from pon farr when he looks like he could be in his teens, but with aliens one can never tell. Perhaps he was the equivalent of a seven-year-old Vulcan there, or of a 28-year-old?
"Every seven years" may well be a coarse approximation and average, too. Whose years are those anyway?
Whether pon farr bothers males who already have mates is difficult to tell, because Trek in general (or TV in general) isn't all that keen on showing happy and ongoing marital relationships.