Seems to me if you down enough alien craft within a certain time frame you should get a period of relative calm. The background says that the aliens have to prepare their ships for atmospheric flight, so one has to assume they can only convert so many at a time otherwise they would just land with everything at once, right? Either that, or they ought to come in waves, like task forces that have been converted and sent all at once. Even then, if you shoot down a task force, there ought to be a period where the aliens have nothing ready to send.