I am no expert by any means, this is actually my first cycle, but from what I've seen 4 weeks it's too short and you hit diminishing returns at 8 weeks. So I figured 6, mainly because the amount I got from my source called for it. I will definitely be doing another 6 week run next year though.
Diminishing returns depends on entirely too many factors to just assume is always the case. How well your body is primed, for instance, plays a very key role in how long the gains are going to flow while on cycle.
There are several ways to combat diminishing returns. Here are my top four ways:
1) Actually prime the body for growth prior to cycling - you can do all sorts of primes but the one I'm talking about here is caloric and insulin-restricting diets. So low calorie or low carb or some combination thereof for enough time to prime the body for growth. Priming is just another way of saying that you have triggered all of the right receptors for growth because you are starving your body to some degree. Ideally, you will also be well below 10% BF for best results. You don't want to be lower than about 8% though, otherwise you will have to reverse diet prior to really eating a high calorie diet because you'll have way too much bias towards the fat cells during that period where your body is scary low in BF. My body is "scared" at 7-8% BF. Most others need to be around 5-7%, IMO. 7% BF should be causing all sorts of striations to appear in the chest, back, glutes and thighs when training. If you have a soft-looking lower belly or buttocks, you likely aren't that lean yet unless you are just weird in your fat distribution.
2) Taper the dosage upwards. Many people recommend against this and at the same time most of them have never tried it. People assume that because the Unitarian approach worked just fine for them,
that there is no need to accept an alternative like tapering. For what it is worth, the guys back in the day did tapering,
especially on the way back down. This is a FAR better alternative for someone who is new to cycling and doesn't want to complicate the process but does want to make absurd gains their first go around. A newbie template can be:
Weeks 1-4 @ 400mg/wk
Weeks 5-8 @ 600mg/wk
Weeks 9-12 @ 800mg/wk
Weeks 13-14 @ 400mg/wk
Weeks 15-16 @ 200mg/wk
Start PCT
Or something to that effect. You could also start lower and not go as high. All depends on what your weight and experience is at perhaps and how long you want to run the cycle. You could also do 5 week increments and only have one increase mid-way thru and then taper back down from weeks 11-14 before starting PCT. I see no reason to have an 8 week run only unless it was prop and ace esters. If you aren't gaining anymore after 8 weeks is not because of the drugs, it is because of your lack of discipline. Just being honest.
3) Change course mid-way thru - meaning change from bulking to cutting or from high carb to low carb or vise versa, etc.
4) Just be patient and consistent with lots of hard work, solid food prep and dedication and make
small,
singular adjustments every week to keep the gains coming. This actually is my top recommendation in any case.