Pipeline Execution Duration

Interested to hear from you, how do you define what is the anticipated run time?
Would you want alerting to notify you of more runtime than usual (absolute numbers, percentages, within a std deviation based on historical data), more documents than usual, less documents than usual, (define usual, should that take into account Day Of Week, Day of Month, Public Holidays (different in every region let alone country)
Do let us know your thoughts.