Estimated time can be added to tasks in a number of ways:
In the tasks area of each project, when you hover over each task you'll see a + icon to the far right of the screen. Click this to directly add estimated time to the associated task:
When creating or editing a task, select the progress and time tab, here you can enter the estimated time against the task:
Viewing estimated time on tasks
You can see estimated time totals at the bottom right of each task list, for all estimated time on tasks in that list:
In the time area of that project, you'll see total estimated time for that project in the panel on the left.
You can also see estimated time included in task list reports, project reports, the Everything > All Time area and the Workload view.
Note - you can also include estimated time when you're importing tasks using the Excel upload option.