Create an instance variable "lifespawn" to your bubbles object.
Either set it to 0 and increment it by 1 every frame/tick until it reaches 120 for example (120 = 2 seconds since the app runs at 60 fps).
Or either set it to your max value, decrement it by 1 each frame and when it reaches 0 then you know you want to destroy that bubble.
In one case or another, make a condition Bubbles.lifespawn = End_Value
+ action Bubbles.destroy (this will pick only the bubbles which lifespawn reached their final value).
This is like the health mechanic in the Beginner's guide, page 6, private instances section