Within Teamwork.com, you can add estimated time to tasks in a number of ways.
In the Table area of your project, estimated time can be displayed as a column option. If a task does not already have estimated time set, hover over the task row and select the Add estimate option.
For tasks with existing estimated time, the estimate will be visible in the cell. Hover over the time and click the pencil icon to edit the estimated time.
Click Add estimate to save the changes to the task.

In the List area of your project, hover over the relevant task and click the hourglass icon to add estimated time.

In the estimated time modal, you can specify the estimated hours and minutes it will take to complete work on the task.

You can also set estimated time when creating a new task or editing an existing task via the Progress & Time tab.

Viewing estimated time on tasks
When a task has existing estimated time, it will be displayed in a number of places within the project.
In Table view, the estimated time will be shown in an Estimated time column on the task row.

Estimated time will be displayed to the right of the task name in List view.

You can also see the overall estimated time total for all tasks in the list at the bottom right of each task list.

The overall total estimated time for a project is displayed in the details panel on the left of the project's Time tab.

Estimated time is also included in task list reports, project reports, the site-level Time area, and the Planner view.
You can also include estimated time when you are importing tasks using the Excel upload option.
For more information, see: Logging Time in the Time Area of a Project