test enth and cyp have active lives around 14 days, but you don't want to inject at that time because that would cause a yo-yo effect with your blood serum levels, a good general rule of thumb is to inject on the half life of the active life, so since test enth's active life is around 14-15 days, you can divide that by two, then by two again, which would be around 3.5 days
this is very simplified by will work for most of the things we work with