That depends on what the half-life of testosterone enanthate really is. There are differing values out there produced from research. I suppose the true value is probably about five or six days, give or take a day.
So let’s say 6 days for the sake of having a figure.
So, ignoring uptake time, 6 days after your last injection, levels are half what they were from your ongoing 600 mg/week usage, or at about the 300 mg/week level. Another 6 days and they are at about the 150 mg/week level.
Another 6, at about the 75 mg/week level. Another 6, and probably at or below the 40 (I could have said 37.5 but that would be overprecise) mg/week level.
At that point you could say it’s close enough to say that blood levels are no longer much affected by the previous injections.
So about 24 days if 6 days is the true half-life, and if really wanting the remaining levels to be quite low.
From the standpoint of not being too much inhibitory, about the 2 week point the inhibition would be about 50% or less (had there been no new injections) so from the standpoint of wondering when recovery would be good, the answer is not so long as being 24 days.
Of course, with the added ongoing injections that you plan, inhibition will still be pretty substantial after 2 weeks. At that point, instead of being at somewhere around the 100 mg/week level in terms of what is in the blood, you’d be at about the 200 mg/week level, which is quite inhibitory.
So I wouldn’t call it a 12 week cycle: what you plan is going to be quite inhibitory for at least 14 weeks, really more like 15.
You won’t have extra gains for those extra weeks of inhibition.
But hey it’s “the thing to do” so of course the practice will continue.