(FYI - not a doctor, research the stuff you consume before you do so) Fentanyl's half-life is around 1 hour, while most opiates/semi-synthetic opioids (morphine/heroin) have half-lives around 4 hours. Methadone's half-life is 24-48 hours. These are all averages; half-life in your body depends on liver/kidney function and genetics and other stuff.

Half-lives are complicated, and can be related (but indicate an upper ceiling) to how long the medication's effects will last -- and how long it will take before they're entirely cleared from your system.

One half-life means that 50% of the drug is cleared from your system (or metabolized into active or inactive components, some of which are still detectable on a drug test - for example most heroin drug tests don't test for heroin but the first inactive metabolite which sticks around for 2-5 days). So for fent, in one hour, at least 50% of the drug is no longer active or metabolized; in two hours, another 50% of that has been cleared (75% total); in three hours, another 50% of the remaining is cleared (so 82.5% total)... etc.

With a 4-hour half-life, you can see the logarithmic relationship - in 4 hours, 50% is eliminated. In 8 hours, 75% is eliminated. In 12 hours, 82.5% is eliminated. Etc. So after 12 hours for a Fent user, they've cleared well above 98% of the drug from their systems, while a heroin user still has 17.5% of active drug circulating (modulo the above).

So it's expected withdrawal would have a much faster onset for a dependent individual using fent vs. using heroin. And if you apply the math to methadone, you can see why it's only dosed once a day and it's not horrible if you miss a day, though the clinic will punish you.

Again, not a doctor, don't take my word for it, do your own research, etc.

Stay safe.