Time To Reply required the following permissions to be granted depending on which option you choose to use to connect your mailboxes to Time To Reply.
o365 Bulk Add
Time To Reply asks for: offline_access, user.read.all, directory.read.all, mail.read
Permission reference: https://developer.microsoft.com/en-us/graph/docs/concepts/permissions_reference
User.read.all: Allows the app to read the full set of profile properties, reports, and managers of other users in your organization, on behalf of the signed-in user.
Directory.read.all: Allows the app to read data in your organization's directory, such as users, groups and apps, without a signed-in user.
Mail.read: Allows the app to read email in user mailboxes.
o365 Individual Add
Time To Reply requires: offline_access, user.read, mail.read
Gmail Individual Add
Time To Reply requires: readonly