This can be set in the Azure B2C AD portal.
- Go to https://portal.azure.com/ and click on Azure AD or Azure AD B2C.
- Click on "User Flows".
- Select the correct UserFlow.
- Click on "Properties".
- Change the session behaviour to the correct timeout.
- Set the number of minutes in: Web app session lifetime (minutes)
- Set Web app session timeout to "Rolling", this way the timeout refreshes when a user is using the portal.