Like when does it make sense to have a business representation of your data in a language like F#? I'd say as soon as you want the full expressive power and safety of records and discriminated unions. Our domain representation was substantially different from what would easily fit in a relational database, so we start with it from the beginning.

In C#, I think ORM classes have a lot going on, so I prefer to use them only for converting to a data structure.