I have extensive experience in this space; I do a lot of work for a couple fortune-5 companies who operate their data centers like an ISP would to the various company departments needing hosting & support services.
They typically have two metrics called an SLA (Service Level Agreement) and an OLA (Operational Level Agreement).
SLAs are met through the type of hardware in use. When talking about SLAs we use levels to describe them. SLA-1 being zero down time, SLA-2 is something like up to 1 hour of downtime, SLA-3 is 8 hours, etc... SLAs are met through the use of redundant equipment. At one company we use a lot of Cisco to create high availability (Cisco CSMs and GSS gear). When talking about SLA levels we generally talk about HA (High Availability) and DR (Disaster Recovery). In situations where a company has multiple data centers, the HA component is usually a per data center attribute while the DR is an across data center attribute; both measured in terms of RPO (Recovery Point Objective) and RTO (Recovery Time Objective) to meant the SLA level.
OLAs are, in real basic terms, how quickly someone (a human) responds to an event requiring manual intervention/corrective action. OLAs are typically measured in terms of response times too; they use the same RTO/RPO objectives. One company I consult for uses 6 levels for their OLA metrics. The first 3 levels here are an example of this:
OLA-1: RTO 0 < 2 Hours
OLA-2: RTO >= 2 & <= 4 hours
OLA-3: RTO >= 24 hours & <= 30 days if not a data center failure, if dc failure > 30 days.
The things that drive OLA and SLA metrics is something called a CIA rating. CIA = Confidentiality, Integrity, and Availability. Data for an application should be classified by the business unit paying for said application. The CIA will help drive what the OLA and SLA should be. Each part of the CIA level is given a number from 1 to 3. So, for example, a CIA rating of 1-1-1 would be Highly Confidential, Highest Integrity level, and Highest Availability level. A CIA rating of 3-3-3 is the lowest you can go. Thus, a CIA rating of 3-3-3 typically maps to an SLA & OLA level of 6 where an SLA-6 & OLA-6 is the lowest (longest response time) guaranteed.
How you derive a CIA rating usually amounts to figuring out how much money a business will loose if the data is stolen (Confidentiality), compromised (Integrity), or when systems are down (Availability). So a company that stands to loose $10M if confidential data is stolen may have a C rating of 1 or if that lost of data is not critical and would only cost the company, say, $1,000 then you may have a C rating of 3 instead.
This is typically how big companies that I've consulted for handle such things.