I'm not sure if any of the spawn logic changes after 300 minutes or so, but it doesn't seem to be getting more difficult. A suggestion that would limit how long people can last is by reducing the time between the carryall drops so that the players can't send out harvesters between waves for as long.
edit: to 660, same thing. more saboteurs, faster waves, more missile tanks in waves, etc, would all make it tougher at this point.
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