The half life of enanthate, which I see quoted the most, is 10.5 days. Some have said that it is more like 5 days which is why you shoot monday/thursday. Since I believe it is 10.5 days shooting on a schedule like that or even every 5th day will keep the blood levels stable.
Thus PCT - post cycle therapy - is started 2 weeks out since 50% of the compound will be in your system at that point and you are trying to stimulate your body into resuming production of its natural T.
There is no true definitive answer that says it will work better for YOU if you do it on week 2 or wait until week three, but clearly you want exogenous levels to start dropping before trying to coax your body into producing T again.