If you’re unsure how long a task or project will take, the best approach is to be transparent with your client from the beginning. Let them know you’re estimating the time and will keep them updated as you progress.
A good method is to set milestones. For example, check in after 30 minutes, 60 minutes, etc, to keep communication open and help the client understand how the work is progressing.

Tip: If possible, spend 15 minutes on the task first and then use that time to estimate the full scope. This works especially well for repetitive or structured tasks like data entry.