I replace mine every couple months, and I shave (my legs, which are pretty long) almost every day (4–5 x/ week).
I know it’s time, not because I’m exactly “nicked” but more because it kind of stings when I draw it across my skin. Also I have the kind with the “conditioning strip,” and when it’s completely empty often coincides with the stinging feeling.
As far a comparing razor options, I think in the end, disposables cost more per blade. Sure a 1 blade disposable razor costs less than a 3 blade razor-head, but they’re not really the same thing.
I fork over the dough for the mach 3 turbo, or reg. mach 3 when i’m feeling more broke, but i don’t go all out for the ‘fusion’.