Throughout the game, occasionally your contact will state "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 is soon to end. Each wave lasts for a about five minutes, depending on how many officers you take down. If you need to complete a time consuming objective, such as making sure the saws in Panic Room are going, camp somewhere near there so you can hold out easier and get the job done. 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.