Depends on the
juice (PG/VG ratio, flavor, sweetener). Basically higher PG liquids, and or light colored juice will last longer. Darker or Higher VG juices are harder on the coils/wicks.
When the flavor gets bad or weak, unscrew the top mouthpiece, then unscrew the ridged metal bottom piece (not the smooth metal piece). The ridged metal piece holds the coil assembly. Pull that out and rinse all parts under running warm/hot water until the wicks are clean. Do not lose any of the O-rings on the mouthpiece or the white rubber piece on top of the coil assembly.
Set aside on some paper towel to air dry, usually overnight is enough.
Continue rinsing them until they no longer produce vapor. Then you can change the coil Usually a few weeks, depending on the juice.