If your snapshot utility is based on volume copy technology, you need a snapshot volume as big as the source volume for each snapshot you want to create, which can add up very quickly. However, if your snapshot utility only tracks changed data blocks, the size of your snapshot volume is dictated by the change rate of your data (volatility). For example, a daily data change rate of 10% on a 100 GB volume requires 10 GB of disk space per daily snapshot.
As for the second part of your question, keeping a snapshot of a volume for more than a week makes sense if there is a business requirement for it. If your organization needs the ability to quickly restore point in time versions of certain files without having to go to tape, multiple snapshots may be desirable. That said, some thought should always be given to disk cost as well as offsite storage requirements for cusiness continuity and DR purposes. It may not be worth the storage cost to keep many snapshots on local disk for extended periods; remote snapshots could address multiple copies and offsite storage requirements at the same time.
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