There is no fixed figure. With regular use, THC is stored in fatty tissue and gradually released back into the blood. That is why THC can still be measurable days after last use, even if you no longer feel anything.
For the 3.5 ng/ml limit, the professional societies for traffic psychology and traffic medicine estimate: people who use moderate amounts on several days a week are usually no longer above it after 3 to 5 days. With daily use or use several times a day, driving is generally ruled out and only conceivable again after a break of several weeks.
How long THC remains is shown by a study of 30 men who had smoked cannabis daily and then used nothing for up to 33 days in a clinic. On admission, the median was 1.4 ng/ml, with a range of 0.3 to 6.3 ng/ml. Of the five who stayed for 30 days, two still had measurable THC at that point (0.3 ng/ml). Measurements were taken in whole blood. The values therefore cannot be compared directly with the limit, which applies to blood serum.
Remember: If you use regularly, you can be above the limit for days without noticing anything. If you use daily, you should not drive at all.
More on this: What does the 3.5 ng/ml limit mean? · How long after smoking weed can I drive? · Road traffic and the law
Sources
- DGVP and DGVM (2024): Empfehlung einer Wartezeit nach Konsum von Cannabis vor Verkehrsteilnahme
- Bergamaschi et al. (2013): Impact of prolonged cannabinoid excretion in chronic daily cannabis smokers’ blood on per se drugged driving laws. Clinical Chemistry 59(3)
- Straßenverkehrsgesetz (Road Traffic Act, StVG), § 24a