Answer
About 22 minutes per month. In review, anything from 14.7 to 33 counts as right.
The worked estimate
30 × 24 × 60 = 43,200 minutes. The budget is 0.05% of that: 43,200 × 0.0005 = 21.6 minutes. Half of 99.9%'s 43 minutes, and five times 99.99%'s 4.3 minutes. One slow rollback can spend all of it.
Numbers: Numbers to know.