Hi Megamanzx and thanks for the question.
In the style of the early 80s arcade games, once each wave of aliens is cleared, the player progresses to the next wave until all their lives are up. So the length of the game is dependent on the skill of the player, and there's no cap on the number of waves and no "final level".
Each new wave is more difficult than the last, with more aliens that also become faster, meaner and more determined, firing increasingly accurate shots.
A novice player's game might only last a minute or two, but more adept players might be able to make a single game last for 30 or 60 minutes.
Hope that helps!