Hover your mouse over the current estimate time (which at first creation is defaulted to 0 hours) and click when it appears. In the input field, enter in a number or use the up and down arrows to add/edit the estimate for the task. Click or press enter to set the changes, or click or outside the edit area to cancel changes. Estimates can only be integers.
Users will need view and edit permissions in tasks to perform this action. Depending on the user's project permission settings, they may be able to view all project tasks, or just a subset. Watchers specifically added to the task will be able to view it.