Time begins when you click the mouse to confirm "you can read." Time ends when you complete the last action required to reduce radiation 6/6.

Example 1: if the last action is breaking stuff in the Service Module, time will end after the last machine is broken. Example 2: If the Mayonnaise Module is your last, time will end as soon as you escape the turbine room and the cut-scene begins.

In each of these cases, time ends when the player doesn't have to DO anything else for the chapter to be over. Have fun!