Answer
About 43 minutes per month. In review, anything from 28.7 to 64.5 counts as right.
The worked estimate
30 days × 24 h × 60 min = 43,200 minutes. 0.1% of that is 43.2 minutes. For reference: 99.99% allows about 4.3 minutes a month, 99% about 7.2 hours.
Numbers: Numbers to know.