A fat-loss phase could technically last a very long time, even over 12 months and you will still continue to make progress. But there's a big caveat: longer phases are best reserved for people with a lot of fat to lose; well over 30% body fat. If you’re already lean and aiming for very low body fat levels (typically below 10%), a fat-loss phase should last 16–20 weeks. After this amount of time, you will notice a significant increase in overall fatigue which would require take a maintenance/diet