Teachers sometimes set time limits for activities or tasks and so long as the time limit is realistic it is thought the limitation creates a healthy level of pressure for students to work under. The time limit is usually set after pairs / groups have been formed, the task has been explained and the instructions checked. It is also common for the teacher to give regular updates of the time remaining for a task, for example during a 10-minute task a teacher might announce the time remaining at 3-minute intervals to remind and motivate the learners.
There are different methods of timekeeping which will suit different learner ages and also activities. Generally a clock or watch will suffice and if learner autonomy is to be encouraged for a task, then the students' own mobile phones or watches can be used.
However, for young learners and very short activities sand clocks can be used. If an activity is to be completed extremely quickly, counting 5 seconds with an outstretched hand is very effective and visually engaging.