If the motor is not being used all the time then you will get lots of flight time, it’s a glider! I would estimate a max constant motor running time of about 8-10 mins with that battery, based on cruising throttle levels, with the motor below.
540 motors are old, heavy brushed motors. What is the all up weight of the plane? I would guess a 2212 1200kv will be more than adequate with a 9” folding prop for most lightweight gliders but I haven’t built one.