One way would be to set the lifetime property to for example 2 seconds and increase the explosiveness parameter to force the particles to all go out together.
Another way is with a Timer:
Set the one_shot property of the particle to on. Then from code you can use the emitting property to turn on the particles. Every time you use for example
$ particle2D.emitting = true the particles will start. When one_shot is activating, they will turn off when they complete their lifetime. You can use a Timer or any other event to trigger this.
I forgot to mention how to use the Timer node: Add a Timer node to your scene, set the desired time to wait_time property and autostart to on. Go to the node tab where the signals are and connect the timeout signal to your script: