Thank you for posting your query on Microsoft Q&A. From above description I could understand that you have MFA configured for your B2C user signin journey, and you want to users who enable KMSI, then they should not be asked for MFA for next 14 days.
Please do correct me if this is not the case by responding in the comments section.
KMSI is places a persistent cookie in the user session, which avoids reauthentication if browser has been closed. (Used only for browser closing scenarios).
It is the Web app session lifetime (minutes) which decides the amount of time the Azure AD B2C session cookie is stored on the user's browser after successful authentication. You can set the session lifetime up to 24 hours and could be extended only when you have a cookie based authentication.
We could not control MFA alone from authentication as it is triggered when Access and Refresh token expires. Each time user is forced to reauthenticate MFA would trigger in. Tokens time could be controlled by configuring the "refresh token lifetime"
The maximum time period before which a refresh token can be used to acquire a new access token, if your application had been granted the
offline_access scope. The default is 14 days. The minimum (inclusive) is one day. The maximum (inclusive) 90 days.
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