[Twan]

It's one VP for the player controlling the province the first month of each summer (month 6 of the year iirc).

I don't know if 3 cumulative victory points is the best choice, I'm hesitating with 2.

2CVP
pro : even if many players manage to control the isle one summer, the game won't be too long
con : if one player takes a good start, takes the isle for the second summer and keeps it two years, the game may end as soon as turn 42

3CVP
pro : good minimum and probable duration (60 turns if a player dominates, in the 80-100 if 2 or 3 others succeed to score some VPs)
con : if all player manage to score vps the game may never end (12 players x 2 vps x 12 months) + 12 for the last vp = ... 492 turns if everyone succeed to control the isle 2 summers before someone scores the third vp)