How to Keep windows 2008 backup for N Days Only

Hi All,

I have windows 2008 R2 Server and I'm trying to set backup of selected folders in to a NAS. Backup work perfect but it keeps backups since I started the backup. Due to this NAS filled and backup stopped. Is there any way I can set backup only for 7days? I check "WBADMIN get versions" and saw 14 snapshots. Is there a way i can delete old snaps? 

I do not want to start a new backup every week as it takes 2-3 days to create initial backup.

Note: 

WBADMIN get versions

wbadmin 1.0 - Backup command-line tool
(C) Copyright 2004 Microsoft Corp.

Backup time: 15/07/2014 2:00 AM
Backup target: Fixed Disk labeled (Z:)
Version identifier: 07/14/2014-19:00
Can recover: Volume(s), File(s)
Snapshot ID: {b9f526df-0db1-44b0-b38a-0f1e38d9d3fb}
......

Backup time: 29/07/2014 2:00 AM
Backup target: Fixed Disk labeled (Z:)
Version identifier: 07/28/2014-19:00
Can recover: Volume(s), File(s)
Snapshot ID: {1d940889-7bf2-467d-85a5-a29e2b98dc42}