It's called bait-and-switch.
It's not a good thing to do. Neither is bringing out quests that people complete and then look at the time it took, look at the XP report and think .... man, I'm never doing that again.
We don't need any complex formula to work these things out. Take a group of their own internal QA people who I'm going to state are average players, time them through the quest. Let them do it 2 or 3 times with no dev hacks. Use their time to set it to 1k/min. Veterans will be able to do it in 2k/min, maybe quicker but so what, you've set the bar where the average player should be.