Don't count out 13,15, and 17 weeks though . . . or maybe stopping your last shot so that it is 13.5 or 15.5 weeks.. not like even numbered cycles do anything better than an odd weeked cycle...
I'd say plan to have enough for at least 16 weeks and then if you are happy with your gains at 12 weeks, stop, go another week or two and if you are still gaining and want to continue, then by all means continue, stop whenever you feel you have met your goals or for any other reasons (sides, personal, whatever). Have a plan, but don't feel like you must stick to it (going the full x weeks), however if you are running test along with it, run the text at least an extra week past the EQ so that way they wear off around the same time or at least so the test wears off last to aid in PCT.