From what I gathered through studies and feedback from several transgirls, it is best to inject estradiol every 7 days (if it is estradiol valerate) and every 10-14 days if it is estradiol cypionate, to not experience the lows, as levels tend to really crash thereafter. This may also vary from one girl to another, metabolism, stress levels, etc. One girl would inject every 5 days while someone else would still be fine after 10 days. But, general rule is 7 for EV and 10-14 for EC, as far as I know.