An SLI is a measured indicator such as successful request ratio or latency. An SLO is a target for that indicator over a time window. An SLA is an agreement with consequences if commitments are missed. Keep these terms distinct.

For a 30-day month, 99.9% availability permits about 43.2 minutes of unavailability if measured continuously using that simple model; 99.99% permits about 4.32 minutes. The real calculation depends on the SLI definition, excluded events, and measurement method.

An error budget is the allowed unreliability implied by an SLO. Teams can use it to balance feature delivery and reliability work. Do not optimize availability at any cost: the goal is a level that meets user needs while preserving sustainable delivery and cost.