How long does chlorine last in pool water?

It depends on a number of factors including how much chlorine is consumed in doing its job of oxodizing radicals in the water. If the water is very well sanitized and nobody is using the pool, there's not a lot for the chlorine to do and of course it will last longer. On the other hand, if the pool water has algae and/or lots of swimmers, there is a lot for the chlorine to do and it will be used up more quickly. Water temperature is also a factor. In cold water not much wants to grow, so the chlorine is not used up in oxodizing things. Sunlight also destroys chlorine - that's why it is advisable to use a stabilizer (conditioner), which helps prevent the sun from destroying the chlorine. The bottom line is, test the chlorine level regularly and add chlorine when and as necessary.