Interestingly, there are no hard numbers that I'm aware of. This may be because so many people don't consistently test their backups. Also, there are a lot of failed backups that tend to go unnoticed.
I would say that if you're experiencing less than a 5% failure rate, then you're probably in the middle of the pack. If you're seeing upwards of 10% or higher failure rates, then it's time to step back and look at your backup procedures, hardware that may be failing because it's old, and data backup system configurations that may not be properly tweaked. This will not only get the highest-quality backups, but will also allow for better monitoring of your system. In turn, you'll be more able to avoid failures in the future.
This was first published in July 2009