Having never tried heroin I cannot make a personal comparison. I can say that I have vaped a fair few grams of 3-FPM. Basically the session almost always ends when the drug does and every bit of residue has been recovered. You sit down to vape one dose with someone and, before you know it, it's 4 hours later and you're nearly out of foil.
I can also attest that I have either partially or fully lost control of myself and my behaviour each time the 3-FPM runs out. However, once that had passed, I had no desire to order anymore..but did as I was ordering for more than one person to build up those points and get the discount and when they were getting 3-FPM I couldn't resist a wee dabble.
I found MPA to be very fiendish for constant dosing, also - even though it didn't really seem to have much effect except wakefulness.