We want to dig deeper into the behaviour around app usage. One of the dimensions we want to use is return frequency to see when visitors come back to the app. The shortest value within the dimension is "less than a day". Is there any way to add more values that are shorter than a day such as within a certain number of hours or even minutes?
Return Frequency buckets as far as I know are not customizable... these are default buckets that are provided to all customers.
That said, while it wouldn't give you the time between visits; you could use the number of visits within a day to get a deeper understanding.. Visits has a default timeout of 30 minutes... so if a user triggers multiple visits within the same day, you know that it's been at least 30 minutes since their last analytics hit....
Or if you really need detailed data, you may have to use a Raw Data export, process the data and come up with a fully customized solution for your needs.