Depends... How long has the tank been up and running? I test daily while cycling, every other day once cycling is over for the first week or two... if everything looks good, I test the day after my pwc every week. If there is any reason to worry, I would do a follow up the next day and so on. Just dont get the mentality that you only need to test if something looks wrong, because generally if you see signs of something wrong, you've waited too late.