Battery life is dependent on how you use them, and also store them. Using them harder (more current draw), and discharging them to more lower service life. Also to a lesser extent storing them fully charged. My P4 batteries self discharge to 65% after 10 days, which is DJI's recommendation for storage charge level. I would concur with the 30% recommendation, but of course that reduces the utility of the drone. If you get 50 flights out of a battery, that's about $3/flight. I would expect 100 or more flights, but some of the pilots here have reported getting less than 50 flights on a battery. I would also concur with removing the battery from service if it shows any signs of swelling. I check my batteries as part of my preflight.