How To Get Average Number Of Days Between Orders

SQL 2012

I'm wanting to get the average number of days between orders in my orders tbl - so I've done a search and found the following sql coded that I have modified for my db tbl's and columns. But when I try and parse it - I get 'Incorrect syntax new the keyword Group' - what am I missing.

SELECT custId, AVG(invDate - priorDate)
FROM(SELECT custId,invDate,LAG(invDate) OVER (PARTITION BY custId ORDER BY invDate)as priorDate
FROM orders)
Group BY custId