I guess it all depends on what your cutting and the size of the blade your using. For me personally blade life depends on a lot of factors, wood being cut, thickness of the wood, size of the blade, speed of the saw, the tension of the blade, and.
If you find yourself having to "push" harder than usual then it's time to change the blade, I will say I find it amazing that your blade lasted as long as it did. I buy my blades by the gross because if I'm cutting hardwood it's not uncommon for me to go thru several blades on a project.
So I guess the best answer would be if you find yourself "pushing" harder than you feel is necessary it's time to change the blade.
Good luck, the puzzle looks great!
Chris