Work item time estimates are now part of Karbon's Time & Budgets functionality. This functionality is available for customers on the Business and Enterprise plans, but it is not part of the Team plan.
If you are on the Team plan however, you add an estimate for Fees for a work item. This is found on the Details tab of the work item.
Note: If you want to discuss upgrading your plan so that you can use the full Time & Budgets functionality, contact us via chat on Help & Feedback or email us at firstname.lastname@example.org.