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Two things.
If you want some configuration to set a point of despawn (usually for low end pc), you can set it on the configuration menu (between 30 - 200 meters of celling).
By other hand, if you want something like "keep the enemies coming, until i blow!", you can enter in Hard mode, fight until the round 8 or 10, and don't blow up the blimp.
While the blimp is still alive, the enemies won't stop coming.
Note 1: those aren't the same thing as an "infinite mode", or a "sandbox mode", but they are closer to something like that.
Note 2: The "hard mode" was not tested before release, so... yes, it's quite unbalanced in terms of resources needed... in round 10, you can even see the frames.