Basically never intentionally. I never say "hey, save copies of important stuff somewhere else just in case" because important stuff is naturally going to make itself at home on at least my office computer, my usb drive, and one of my home computers. So if any of those things fail, the worst possible loss is maybe one day's worth.
At work I've got one exception, my QuickBooks prompts me to back up every three sessions, which translates to at most every three days. This is sent to a dedicated external hard drive that is in turn backed up every month or so. Losing even three days, let alone more, would end up costing me at least twice that much time to restore. My job is boring enough as it is, I don't need to do the exact same repetitive paperwork over again. I should probably switch it to back up nightly, considering how frustrating it is to lose even an hour.
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