You should cut (go into a fat loss phase) primarily when your body fat percentage is too high, or if you are overweight or obese and need to improve your physique and health. Specifically, cutting is recommended for men when body fat is above around 15-20%, and for women above around 25-30%, though these numbers vary slightly depending on sources and individual goals. The purpose of cutting is to reduce body fat while maintaining muscle, typically through a calorie deficit. It is advised to cut if you want to be leaner before focusing on muscle gain, if you have gained an uncomfortable amount of fat during bulking, or if you have health-related reasons such as reducing risks linked to excess fat. Key guidelines on when to cut:

• If body fat is high (men >15-20%, women >25-30%)
• If overweight or obese
• After bulking when fat gain is excessive
• When preparing for a competition or aesthetics focus
• When health concerns from excess fat arise, such as risks of insulin resistance or cardiovascular issues

You may also stop cutting and start bulking once you reach a lower body fat threshold (e.g., 9-10% for men or slightly higher for women) to avoid being too lean for effective muscle gain. Cutting should ideally be done during times of manageable life stress due to the mental and physical strain dieting may cause.