Average between dates

I would suggest you do this with a calculated column in your order table with an expression like this

Days Since Last Order =
VAR thisdate = Orders[OrderDate]
VAR lastorderdate =
    CALCULATE (
        MAX ( Orders[OrderDate] ),
        ALLEXCEPT ( Orders, Orders[Customer] ),
        Orders[OrderDate] < thisdate
    )
RETURN
    DATEDIFF ( lastorderdate, thisdate, DAY )

Once you have that, you can simply use this as a measure to get your result

Avg Days Between Orders = AVERAGE(Orders[Days Since Last Order]) 

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat