I don't believe this is possible in Adobe alone... if you were to do raw data extracts, you could get the last timestamp recorded for each visit of each visitor and compare to the timestamp of the next visit for said visitor... calculate each timespan, then average it... but that is too complex for the calculated metric builder....
The closest you can get out of the box is "Days since last visit" (if the visitor came back with the same day, it will be "0 Days", if they come back the next day it will be "1 Days", etc...) It's not so much "time between visits", but it's at least a simple gauge of the returns...
Or you can use Return Frequency, which buckets users into groups:
Less than 1 day
1 to 3 days
3 to 7 days
7 to 14 days
14 days to 1 month
longer than 1 month
Just remember that the same user will show under multiple "Days since last visit" or "Return Frequency" for each different span they had between multiple visits.