What about if the time frame I am looking at is 7 days and some users return more than once?
If a user visits monday, than returns wednesday, and then again thursday, that would count 1, 2 or 3 days?
The "?" next to the column "Days since last session" states "The number of days from the close of one session to the opening of another". Based on that and using your example, the answer would be one day since the users last session. You can also look at https://www.nngroup.com/articles/frequency-recency/