Answer
About 53 minutes per year. In review, anything from 35.3 to 79.5 counts as right.
The worked estimate
A year has 365 × 24 × 60 = 525,600 minutes. 0.01% of that is 525,600 × 0.0001 ≈ 53 minutes a year, or about 4.3 minutes a month. Each extra nine divides the allowed downtime by ten.
Numbers: Numbers to know.