+1For me, its when the blade starts pulling instead of cutting.
I have found that when a blade starts to tug ATG it's time to change it.
Once or twice a week.
They are 12 cents each, why split hairs
How do you know when to change blades?
By feel? Will it start cutting the skin?
Plus, about how many(on average.... I know everyone is different with different beards) shaves can you get out of a blade?