This is worth to note.
Availability in numbers
Availability is often quantified by uptime (or downtime) as a percentage of time the service is available. Availability is generally measured in number of 9s–a service with 99.99% availability is described as having four 9s.
99.9% availability - three 9s
| Duration | Acceptable downtime |
|---|---|
| Downtime per year | 8h 45min 57s |
| Downtime per month | 43m 49.7s |
| Downtime per week | 10m 4.8s |
| Downtime per day | 1m 26.4s |
99.99% availability - four 9s
| Duration | Acceptable downtime |
|---|---|
| Downtime per year | 52min 35.7s |
| Downtime per month | 4m 23s |
| Downtime per week | 1m 5s |
| Downtime per day | 8.6s |
For more you can check https://github.com/donnemartin/system-design-primer