Depends on the ribbon and amount of typing. The newer electronic ribbons were a one shot deal as once a character is typed all that is left is clear plastic. The older black only ones could be used over and over again until the ink was so used that it made it hard to read. The 2 color ones (black above red) lasted about twice as long as when the black ran out one could go back and use the red. Different brands came in different lengths so coming up with finite numbers is going to be a bear.
