RAID 0 is the WORST type of storage for data protection - if EITHER drive fails then you'll loose ALL your data, and with two drives the chance of failure is DOUBLED.
RAID 1 is probably the safest. But even RAID 1 won't protect your data from accidental deletion, corruption, theft of your computer, etc. etc. For the best protection, instead of using RAID just take one of your drives and put it into an external USB 3.0 enclosure. Then simply make a manual copy of everything once a week or one a month, then store it separately from your computer.
Well, according to the Google study on hard drive failure rates a particular hard drive has very roughly a 5% chance of failing during any given year. Since RAID-0 with two drives is twice as likely to suffer a failure, we can guess that its chance of failure will be around 10% in a given year.
Is that "often"? It depends on how precious your data is, I guess.
For storage only, you really shouldn't use RAID 0 or 1. Use RAID5 with 3 or more drives for some level of data redundancy, RAID6 for even more redundancy.
Using Mirror RAID as a backup strategy is just bad, as it actually increases your drive failure rate and doesn't protect against corruption issues - like sminlal said, you're better just having an "active" drive and a "backup" drive. Leave the backup drive offline, except when "backing up" data to it. It's really not the same as having a backup, because you're uninsulated against data corruption, but at least you have a second copy of your data somewhere if the active hard drive catches fire or something.