The amount of time a disposable razor lasts depends on a number of factors, including how often it is used, the type of razor it is and how many blades it has. The average amount of time a three-blade disposable lasts with daily usage is one week.
Some of the newer models of disposable razors are made to last, claiming they will last up to four weeks or longer. If you want to extend the life of your razor, try to dry the blades the best you can with a towel, then put the plastic cap on top of the blade between every usage. This will prevent oxidation that often causes a razor to work inefficiently.