It definitely varies person-to-person (depending on how thick your hair is, how often you shave, how much you're shaving, and more)—some swap their blades every 2 weeks, while others will go more than 2 months between changes.
Leaf suggests experimenting to find what works for you. You should change them when you start feeling that the blades are dull (on average every 6 to 10 shaves).
If you do keep your blades in for weeks at a time, give them a deep rinse after every couple uses. Just open up the head of the razor, remove the blades, give it a good rinse to clear out any soap residue or hair, and load the blades back in place. This will keep the razor in peak condition when coupled with a periodic cleaning routine.