Dairyground - normal distance to descend from cruise altitude to destination is 120-140 nm.
Emergency, max effort, from max certified altitude, is probably closer to 70 nm.
The time for a normal descent is 25-30 minutes. You can usually estimate your time of arrival based on when the first descent starts assuming normal airspace constraints and arrival flow.
An emergency, max effort, would be closer to 8-10 minutes. Slightly quicker for lower cruise altitudes. If time is critical it doesn't matter if you're over a runway if you're too high. It takes time, and distance, to reduce the altitude.
So being right over an alternate doesn't help much. It's actually easiest if it's 120-140 nm ahead of you when you make the decision to divert and you're ready for the descent. From there you'd just do a normal descent.
Last edited by misd-agin; 4th Feb 2017 at 13:02.
Reason: Added 2nd and 3rd paragraph.