Even more than 5 hours - I've taken the Paris to Milan and Paris to Barcelona trains multiple times (6:30-7h), and both are relatively frequent, quite full, and

Even more than 5 hours - I've taken the Paris to Milan and Paris to Barcelona trains multiple times (6:30-7h), and both are relatively frequent, quite full, and competitive with air travel due to a number of factors even if they're slightly slower (for Paris-Milan depending on departure point and airport on both ends, you're looking at 30-60 mins per direction, plus being 2 hours early at the airport, plus the time to get from the gate to the train station to get out of the airport, plus 1h30 flight and you're looking at 5-6h for the plane vs 7h for the train): * vastly more comfortable, with more space and amenities * nice views * single style of hassle (get to the train, sit down, enjoy; with a plane you get to the terminal, pass through security, then wait, then go in a plane in heavily restricted mode, then you can move/do stuff after take-off, then again you go in restricted mode, then you wait for taxi to finish, then you get to the gate, then you walk towards the exits, wait for luggage, etc.)