Paul, this is a question that there is no accurate answer to, blade life depends a lot on what your cutting. You state that you usually cut MDF, well with MDF it can depend on what type or how much glue was used in making the product. I've had blades break as soon as they hit the wood, but, I've also had blades cut until there was no teeth left. Based on these experiences, how could I ever answer your question correctly. They last as long as they last, there is no set amount of inches or centimeters assigned to any blade. Can you only imagine how many warranty claims there would be, if the blades were assigned length of inches they will cut? Bottom line, get what you can out of a blade, and be happy with what you got.
Len