You can use the water float indicator to monitor the overall water level. When that hits the bottom and you can't bob it up-and-down with your finger then it's time to refill the tank. The water tank lasts for about 2-3 weeks, depending on the plants but you can definitely fill up the tank sooner if necessary.
Please see below the correct float level. Make sure not to add in too much water, otherwise, the plants might get overwatered.