The number of milliseconds this emitter will emit particles for when in flow mode, before it stops emission. A value of 0 (the default) means there is no duration.
When the duration expires the STOP event is emitted. Note that entering a
stopped state doesn't mean all the particles have finished, just that it's
not emitting any further ones.
To know when the final particle expires, listen for the COMPLETE event.
The counter is reset each time the ParticleEmitter.start method is called.
0 means the emitter will not stop based on duration.