Time limits must be prepared beforehand and followed because it can make the difference between getting through everything you wanted to cover in your lesson or running out of time and failing to complete critical components. Make sure that time limits are not given out arbitrarily, they should have been thoughtfully planned out to cater to your students and figuring out how much time they would need to cover a specific component.
After the timer has expired, students should understand that the time is over and they will not receive any more time. If you don’t enforce this, you make your time limits subject to situation and students may feel they are allowed to always ask for more time, which may cause time management issues towards the end of your classes. This may lead to failing to introduce homework, giving feedback to students, or even getting through a component at the end of the class.