Why not run it for 15-20 weeks 500 mg per week? Do the sides increase the longer you run it? If you have enough test is it safe to run it for 15-20 weeks? Same mg the whole time. For beginners of course.
I'm assuming you have never run a cycle before. I hope so at least, because this is an answer you should have sought out before you cycled in case you have. The main reason being that 12 weeks is all a newbie wants to be suppressed for. Just because a newbie has a better ability to recover, all things being equal, than a vet (believe me, I have seen freak vets who can recover fast) we have sadly learned that the body does in fact hate being suppressed. A good chunk of vets on here are on TRT. Sadly, myself included here, a good portion of those guys are on TRT because they chose to do long cycles, short cycles, and all sorts of cycles in short succession. Hey, I love my TRT, it's awesome. Just means I am responsible for maintaining and monitoring my own vitality. For a brain that thinks, and operates like mine, that's perfect and I wouldn't go back. I can maximise it, but most importantly, monitor my BP, E2, and an array of other things. But a newbie shouldn't be cycling 12 weeks because of the suppression factor. All other points made were relevant too.
You will see vets cycling longer. This is generally because they are running other compounds with their test, compounds that require 16 to 20 weeks to be maximised, but also require test to be a base. These vets monitor their vitals, they get blood work, they manage their blood pressure. They are vets and thye know how to do this in the safest manner possible through others mistakes. Gains on test will in fact plateau after about 12 weeks on a short ester test, and 16 weeks on a long ester test. Why keep shooting 300 - 500mg of test a week if it just keeps you normal? Change your diet, come off and work out hard.
If you're a newbie the best cycle you could do is 8-10 weeks test Prop at 300mg / week. With a proper diet and awesome training, with an hour walk in the AM you'd be a ripped beast in 8 weeks. A shell of your former self. But i don't know your age or stats, so I'm sort of talking up my ass.