When you’re dealing with services, a common measure is the percentage of uptime or availability the customer can expect the supplier to provide.

As an example, for downtime per year:
99% ~ 3.5 days.
99.9% ~ 8.75 hours.
99.99% ~ 50 minutes.
99.999% ~ 5 minutes.

Five nines is what this last metric is commonly called. Nine fives would be:

55.5555555% ~ 162 days of unavailability per year.

Which, at less than 182 days, is better than a coin flip.