Percentage Uptime Downtime Per Year
99.9999% 32 seconds
99.999% 5 minutes, 15 seconds
99.99% 52 minutes, 36 seconds
99.95% 4 hours, 23 minutes
99.9% 8 hours, 46 minutes
99% 3 days, 15 hours, 48 minutes
98% 7 days, 7 hours

Table 1 Identifying Your Availability Levels.
What service level do you need to obtain? If you're going for the five nines, then you'll need to make sure your systems are configured properly and tested thoroughly.