System statistics - How often?

Anyone have any recommendations for the collection of system statistics? Any recommendation on how long to run the capture? I've heard 72 hours before (ie, M-W during a business week, which should represent the bulk of activity, shifting the window if you have more activity on X day).

Is there any reason to re-gather system statistics if hardware remains unchanged?

Also, when you want to ensure you can go back to old system stats, do you export, or use a stats table, or some other method?

Thanks - Chris

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