Inspired by this question:
Currently, the only answer there addresses the fact that the "names you first know someone by are hard to shake". However, when thinking on this, I realized that another factor would be the amount of time during which Luke actually knew his mentor as Obi-Wan, in comparison to the length of their relationship beforehand.
Luke came to know the true name of Obi-Wan Kenobi fairly early in Episode IV. At this time, he was about 19 years old and presumably had known (or known of) "Old Ben Kenobi" all his life. So, to get a rough comparison of this against how long he actually knew "Obi-Wan Kenobi", we just need to know how long (in in-universe time) Episode IV was.
Starting from there, I'd like to expand the question to more generally encompass all six of the movies which are the primary canon of the Star Wars universe. So, over what duration (and, ideally, dates) did the events represented in each movie occur?