With most hydro growing methods, check the pH and PPMs of your nutrient solution every time you feed, or every other day. With soil, be sure to check the pH of the run-off solution, too. With drip systems, or to be the most thorough, you should also check the grow medium itself. For instance, with rockwool, squeeze from where most roots are present (middle or bottom of the grow medium) and test pH of the resulting liquid. Be sure to check during the day as levels can vary from day to night.