There is no safe number of hours. How long THC acts and stays in the blood depends on the amount, the THC content, how it is taken and habit. The professional societies for traffic psychology and traffic medicine therefore recommend these waiting times as a minimum:
- At least 12 hours: occasional use, a moderate amount smoked or vaporised
- At least 24 hours: THC content unknown or a larger amount
- More than 24 hours: eaten or drunk
- Usually 3 to 5 days: regular use on several days a week, moderate amounts
- Several weeks: daily or several times a day
Occasional means there are several days between two days of use. A moderate amount is no more than 25 mg of THC, about 0.25 grams of cannabis with 10 per cent THC.
Being below 3.5 ng/ml does not mean you are driving safely. Impairment can also occur below it, and anyone who cannot drive safely because of cannabis commits a criminal offence (§ 316 StGB). Feeling sober is not enough to go by.
During the probationary period or under 21, you may neither use while driving nor set off under the influence of THC (§ 24c StVG).
Remember: Occasionally and a small amount: at least 12 hours. If in doubt, longer. If you use a lot every day, you should not drive at all.
More on this: Road traffic and the law · What does the 3.5 ng/ml limit mean? · How many nanograms of THC do regular users have in their blood?