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For all subsequent logins, the login process prompts users to supply a registered method in addition to their username and password.
If you’re using Salesforce's MFA functionality, users must respond to an MFA challenge each time they log in to a Salesforce product. This applies to all logins, including those due to inactivity and expired sessions. The frequency of MFA challenges can’t be modified.
Can I automate or control how often the extra authentication step is required by Salesforce products to reduce impact to my users?
To ensure that MFA is providing the intended protection, users must supply a verification method each time they log in directly to a Salesforce product. To reduce friction for users, we recommend using Salesforce Authenticator. The app can automate the extra authentication step when a user works from a trusted place, like the office or home — which means users don’t have to touch their phones when they log in from these locations. Users can set this option for themselves. See Automate Multi-Factor Authentication Logins from a Trusted Location with Salesforce Authenticator in Salesforce Help for details.
Note: FAQ on how MFA works and how it will affect user experience are available at https://help.salesforce.com/s/articleView?id=000352937&type=1