IMO for best performance you should have a power feed about every 8 to 10 track joints, irrespective of how long the track pieces are. Especially if you are running conventional, so as to avoid slow downs in the corners, which is very annoying. With command operation you don't get this problem. The track joints are where you will get the most discontinuity and therefore power loss. If you are wiring for dcs the recomendation is every 12 track joint maximum, I believe.
In the end regardless of what you choose, you can always add more feeders if you have any bad areas. 
Rod
