How long do you have to wait to change the zero water filter?

You know that brand of water purifier, zero water well I was wondering how long you have to wait to get a new filter because the little tester thing that comes with it broke and doesn't read the amount of metals in the water accuratelyHow long do you have to wait to change the zero water filter?maybe you should get a new filter since the tester broke so you wont be guessing all the time and wasting money by buying too early... but id say once a month... whatever you did before it broke..