How to convert the following query to give sum per week

This is part of a long query which compute the total time per day. I want to change it to give me the total time per week. Any idea? To check the long query click Here

INSERT INTO @test (
    activity
    ,DATE
    ,TimePerDay
    )
SELECT activity
    ,Cast(starttime AS DATE) AS DATE
    ,SUM(datediff(second, starttime, endtime)) / 60.0 AS TimePerDay
FROM cte WITH (NOLOCK)
WHERE starttime >= dateadd(week, @LastXWeeks, last_update)
GROUP BY activity
    ,cast(starttime AS DATE)