I get a little more time out of my Spark Batteries, like probably 12 minutes. DJI claims the flight time is 15-16 minutes, but that would be fully draining your battery which is not good for it. Only a few times have I risked my drones by pushing the battery this far.
Of course the easiest way is to purchase more batteries, would require returning to home point for a battery change. For me I have had previous drones that would last 1/3 of this time so the Spark was a great improvement and more than I need for my purpose.
However I have seen mods on Sparks that allow them to take standard LiPo batteries to give them extended range. They are not common, and may be difficult to find since the spark is not mainstream anymore.
I have a used Spark with 6 batteries and different recharging cycles (from 25 to 60). In cold weather they last 6-10 minutes: I'm waiting for sunny days to see if I can fly at least 12 minutes!