tsql - How to calculate average date occurrence frequency in SQL -

i'm trying produce query on following table (relevant portion only):

create table [order] (   orderid int not null identity(1,1),   creationdate datetime not null,   customerid int not null ) 

i see list of customerids each customer's average number of days between orders. i'm curious if can done pure set based solution or if cursor/temp table solution necessary.

;with base      (       select  customerid,              row_number() on (partition customerid order creationdate, orderid) rn          [order]      ) select   b1.customerid,          avg(datediff(day,b1.creationdate, b2.creationdate) )     base b1          join base b2          on       b1.customerid=b2.customerid          ,      b2.rn        =b1.rn+1 group b1.customerid