This is a confusing matter for all in this business, the half life of any ester can't be accurate because it's an individual thing it varies upon individuals and sites of injections (or muscle group), what's know about cyp through all these years is that it has a half life of 12 days, but i think this is wrong and not accurate at all, you know cyp and E both usually (or statistically) peaks arround 36-48 hours and drop below baseline arround the 4th and the 5th day, it's better to shoot them twice a week to keep levels stable, as for how long for half of the drug is released it can vary from like 5 days to even a little more than 12 days depending on ur own metabolism and site of injection. so don't worry about that just injected twice a week and u'll be fine , you may be fine with once a week too you're the only one who can figure this out by your bodies response, a 7 days gap is not a big deal for most.
Little more background info below...
center4menshealth.com/PDFs/Myth_2.pdf