This very much depends upon the quality of the blades you are using, if you are using the correct blade for the board and the abrasiveness of the passe-partout. As a guide for good quality work I would suggest 5 – 8 from one blade tip dependant upon the size of the passe-partout. However, with a little experience you should be able to feel when the blade starts to get dull and change it accordingly.
I have come across a number of framers who change the blade every passe-partout on the basis that the cost of the blade is negligible compared to the cost of a passe-partout not being right.