Most places I have worked have an 'over X hours' policy for business class. A couple have had 'no business class within North America' rules, regardless of duration. There are sometimes exceptions for internal meetings, and I've known specific airlines business class be excluded because their cost was so much higher than other airlines (Emirates in the case I recall). Many places also have a 'prudency' policy - if you knew you were going to a meeting in Sydney 3 months ago, but waited until the day before to book your (now full fare) flight, without good business reasons, you would be hauled up to explain yourself.

I've also known companies have policies on the type of fare - i.e. no full Y or the fares that basically get you an instant upgrade to business, despite being technically economy fares. Depends how savvy the expense / travel department are!