Storage mode charging has an automatic cutoff, when storage voltages are reached, just as it cuts off after balancing if full charge mode. PLEASE READ: You do not simply leave them on the charger. The battery is removed and stored in their flame retardant bags, as it should be at the completion of any charging cycle.
How long can the batteries be kept at storage charge? Best practices here suggest that the battery chemistry will degrade at an increased rate, when not in use. When possible I would take out any storage batteries that are reaching the one month mark, charge to full, and go fly them out for 15 minutes.
And now for reading all the way through... a little shortcut thru the woods, for those that have trouble keeping track of all this.
If you take the stored batteries once a month, charge up, and go fly the H... if you land at a target voltage of 14.4 - 14.5V... when you bring those batteries back home and they have cooled, the bounceback voltage under no load, will be within 0.1V of the recommended storage voltage of 15.2V. Easy, peasy... just put them away.
For those who feel that 0.1V leeway is too liberal, I agree... but at that point you have no business using the DY series anyway. You should be using a 3rd party charger to measure the truly relevant measure of battery health... internal resistance. No drone manufacturer provides those... even ones that claim their batteries are "smart".