The half life of Phenibut is around 5 to 6 hours but for some people the effects can last all day.
Phenibut should kick in within 1 to 2 hours and can take 5 to 6 hours to peak and fully take effect.
Also your results may vary and the time it takes Phenibut to kick in will depend on the amount of food that is in your stomach, your metabolism, the dosage, and how you took it and taking Phenibut sublingually results in a faster onset time than if you were to take it with some drink such as juice or water.