Do you treat every incident or trouble ticket as if each one has the same priority? Probably not. After all, there is a lot of difference between resetting a password and having a product or service  go down completely.

So you differentiatie the incidents according to.....what? Probably how much impact it has on your customer. One call center where I worked we had level 1, level 2, and level 3. However, each of those priorities had more to do with whether customer support/field service was able to resolve the problem and whether it was a major issue impacting patient or operator health. It wasn't really related much by time, although the quicker the better.

What you really need is something that tells you how important the issue is to your customer and how quickly you need to address it; urgency and impact. I consulted Kristin E. Robertsons' book Spectacular Support Centers  for best practices in establishing priority definitions and responses. 

Most call centers will have 3-5 priorities. Any more than that and there is not enough difference between the priorities to make a difference in response. While each industry has its own nomenclature for this type of classification, I will use some basic terms for the various levels. The response and resolution times will differ according to the industry and the impact on people or business. Reponse to a Critical incident might be only 5 minutes where health is impacted, while 2 hours might be sufficient for a less dangerous but still needed process.

Critical

• Highest priority
• Definition: Production halted, unplanned outage, work stoppage, VIP customer, or an impact to corporate revenues or image
• Examples: email outage; employees uploading a video with negative content a la Domino's Pizza. 
• Response:Immediate contact, even after hours

• Definition: Anything keeping an employee from doing the job and there is no work-around
• Examples: Software module unavailable, a slow centrifuge
  
• Response: Within 8 hours

• Definition: Anything impacting a significant part of user's operations. Production is hampered but work can contiue but deadlines may be missed.
  
• Examples: Printer malfunction, 1 of 3 washers has overflowed.
• Response: 24 hours
  

Low

• Definition: "Annoyance", nothing untoward would occur to the business if it remained unresolved. Does not involve deadlines or work speed and completion.
• Example: password reset, simple question
• Response: 48 hours

Planned

• Definition: Maintenance that will take equipment or software offline.
• Example: Database backup, replacement of worn parts
• Response: 10 days

These are some examples of terms, type of problems, and response times that have been used by call centers to manage resources and keep service levels high and consistent.

If you are not currently prioritizing your trouble tickets, you may be wasting time or losing customers. If you are prioritizing but have no official written policy, you need to write one. 

Everyone should be using the same term for the same priorities. Anything else will produce poor results, upset customers, and stressed employees.