Estimating time can be tricky, especially when you are dealing with practical elements in a lesson.
It can be very hard to judge how much time a class will take on an activity.
Too quick and your class could end up frustrated and confused, too slow and your highflyers may get bored. Be prepared to be flexible.
If you know your class well, you should be able to estimate how long they will take to complete a particular task but if there are questions that need answering then be prepared to adapt your planned lesson on the spot.