There really isnt any set amount of time a disposable razor can be used.
Once the lubricating strip starts to wear away and the blade dulls it is time to get a new one. A dull blade will feel like there is much more resistance as you shave and instead of shaving hair you may feel it is doing more to just pull it out. This can lead to more irritation and ingrown hair
Answer assisted by Garrett Caldwell, Medical Student