Time to fix, is the maximum time to resolve the issue, but this is subjective and its not a guarantee depending on several factors.

Let's assume you have a Debian networking issue, we can guarantee a fix to that within 2 hours, except where its a hardware issue, then it depends on how quickly someone can swap cables etc. 

In a certified solution, with a certified TTF then it IS a guarantee, otherwise its a best efforts scale, and an indication of resources assigned to any one issue, and the order in which issues are despatched by the support teams.