With some problem management practices, problems are typically
classified into severity levels, along with expected timeframes for
resolution.  See the example below:
  Severity 1, 90% within 4 hours
  Severity 2, 90% within 8 hours
  Severity 3, 90% within 24 hours
  Severity 4, 90% within 72 hours

Can you provide links that describe industry standards for these
targets?  The primary venue I'm interested in is IT (information
technology).  So white papers, case studies, or even books (if
searchable on-line).