Here's my company policy:

Forget about first altogether (unless it is a Business/First like Continental).

For Business class:

Fly one trip greater than 12 hours on the last leg (no layovers over 12 hours) in coach.

If, within 17 days, you need to fly another leg greater then 10 hours (not the return leg of a previous trip, a whol new trip), you may book business. This is called a mid-length flight.
If you, within 17 days, you need to fly another leg greater then 12 hours, you may book business. This is a called a long flight.

I still never figured out what the difference is.

Oh, and also, this is per business quarter. The 'qualifier' rule resets each quarter.

Overiding the policy is the opinion of the VP (the approver), who will demand coach anyway when the price difference is noticeable (and when is it not?)

The Division President (GM) flies nothing but First because he has a 'bad back', which is likely due to his obesity.

There ya have it.