Throughout the game, occasionally your contact will say "An assault is coming in X seconds." Listen carefully to what he says. It is a hint of special units coming in, and when the assault will begin. Each wave lasts for approximately five minutes, depending on how many officers you kill. If you need to complete a time consuming objective, such as making sure the saws in the Panic Room are going, camp somewhere near there so you can hold out easier and get the job completed. Note: Some missions have "escape" objectives, which means the wave will never end until you either complete the mission or die. You will know when this happens depending on the mission and what the contact yells.