You know the purity of the heroin you're getting?? Really? Lol, fuck no you do not! (no offense)
Yes, typically fentanyl transdermic patches last up to 72 hours, but it should say on the patch or box what strength it is, either way, it minimum it will take away your withdrawals for sure. If I were you, I'd get 1-2 more patches, use those for another week, then switch to suboxone for maybe a week tops, tapering down gradually and
then you'll have very minimal withdrawals if any at all and you'll be clean!! If that's what you want...
To answer other questions, NO, it will not replace heroin, as black tar heroin = diacetylmorphine/6-MAM/unreacted morphine/acetylcodeine mixture and fentanyl is fentanyl but fentanyl is a synthetic opioid that does same thing essentially but is shorter acting. It will stave off your withdrawals, how much so is hard to say based on strength of your dope.
Too bad CC is in a drought, maybe you should move up to DFW, no shortage of heroin up here lol.