About a couple hours after the first dose.
To solve this enzyme problem, use inducers such as Carisoprodol, Phenobarbital, Ethanol, etc.
For these, medium doses are really all that is necessary. You can go from 1 to 10% conversion to morphine.
This will greatly increase the ceiling dose and solve the problems of codeine tolerance for chronic users.
Ethanol is going to always be the cheapest cost effective booster of opiate/opioid conversion.
Anti-histamines will lower the ceiling dose for codeine due to them also disabling enzymes.
So if your codeine is mixed with promethazine, you will not get as high as if you had codeine alone then a promethazine dose 30 minutes to an hour later.