The easiest way to do so is to go to Reporting> Employees Dashboard and find the user and the course/curriculum in question.
Past Completions for Courses:
You can see the number of total completions per course and by hitting the relevant icon next to the total completions you will be able to see also the Past Completions' dates and the time that the user spent per past completion.
Moreover, by clicking on the arrow icon next to the past completion date, you can also the time spent and the number of attempts per course step!
Past Completions for Curricula:
Starting from 12/15/2020, the total number of curricula completions and the dates of curricula completions are stored and displayed on the reporting dashboard.
Please note that if you Obliged a user to retake a Curriculum prior to that date (12/15/2020) his completion will not be available under the "Past Completions" block.