Partly it's observations over the past week. I noticed a few days after moving that I had two backups the day I created my account, and no backups for 3 days after. Contacted support and was told they have some 'randomization for load balancing'. Asked what the worst-case frequency could be, and was told it's "usually 24 hours". This is the kind of thing that can bite you when the shit hits the fan.
(And the experience definitely shows what other commenters said about Linode's support.)