You just need to know how much current the dongle drains off the phone. The EIDA consumes 100mA, other dongles around 25mA. If your mobile phone's battery is rated 3000mAh then it will last not longer than 3000 mAh/ 25mA = 120h. Considering the cell phone does some others tasks while reproducing music files it will last probably half this period. My LG G7 Thinq lasts around 48-60 hrs depending which in-ears I use and always in 2V mode. Way longer than any DAP I know of.What phone? How many charge cycles? What dongle? What headphone? What volume? WiFi/Bluetooth/GPS/NFC/... On or Off? Battery Saver On or Off? What Apps are open in the background?
Basically, it's impossible to answer that question due to sheer amount of variables.