It takes 5.5 x elimination half-life of a medicine before it is eliminated from your system.
Accutane (isotretinoin) has an elimination half-life of 21 hours, therefore it will take 5.5 x 21 hours = 115.5 hours (approximately 4.8 days) before it will be cleared from your system. Side effects may last longer. You must not become pregnant for 1 month after stopping Accutane.

However other factors to consider include:
1. How much and how often you have taken the drug.
2. Your metabolic rate – a slower metabolism will increase the time a drug remains in your system.
3. Your age and health – older age and poor health will generally increase the time the drug stays in your system.
4. Body mass – generally the bigger you are the longer a drug will remain in your system.

For more information see:
https://www.drugs.com/pro/accutane.html