There is another variable, 'Days Since Last Visit' that provides the granular days.
Regardless, to solve this you would likely need to use Data Feeds or Data Warehouse in lieu of Analysis Workspace or Report & Analytics.
The primary reason being that both 'return frequency' and 'days since last visit' will count unique visitors in multiple groups. This is explained within the help docs:
The date range in this report uses the time in which the visit occurred. For example, a visitor comes to your site in April, then comes back twice in the same day in October. Pulling a Return Frequency report for the month of October would show one visit under "longer than 1 month", and one visit under "less than 1 day".