Under 6 hours - Economy+/Comfort/Extra Room/ETC, or Economy if the particular flight doesn't offer an upgraded economy.

Over 6 hours or TATL - Business Class/Premium

Over 12 hours - First

Certain travel billed to the client under 12 hours can be First as well if agreed to in the contract.

Also regarding the discussion above of only highly paid employees being worth expensive travel, that may or may not be true depending on the industry. One of my past companies sent $50k a year technicians on F to client sites all the time and needed them to be well rested and ready to work on arrival.