It is all based on how you are lifting, eating, sleeping etc. And how your gains have been up till now.
Going from 8 to 12 weeks give you another month "on". If you have the gear go ahead and stretch it out to 12 weeks. What are your stats like now? What weight did you start at and where are you at now? BF%?
What are your lifts like starting and now? Dead, Squat, Bench, Mil?
What are you planning for post cycle therapy (pct)? In my opinion you should start your PCT 2 weeks before your last injection.