It’s almost impossible to determine the blade life by how long they are used, especially if you skate on different ice conditions. The best sign to know if  you are due for a sharpening, and by far the most reliable, remains the feeling you experience while skating: if you feel that the blade(s) edges do not grip into the ice when you push to move forward, then it is a clear sign that your blades are due for sharpening. It is always recommended to sharpen your skates on a regular basis for your own safety and to also avoid their damages due to longer sharpening.