The FITT Principle F – Frequency Definition: How often you work out? Recommendation: 3 – 5 times a week. The FITT Principle I - Intensity Definition: How hard you work out? Recommendation: Increase the level of intensity; work in the target heart rate zone to 60 to 85%. The FITT Principle T – Time Definition: How long you work out? Recommendation: 20 – 60 minutes. The FITT Principle T – Type Definition: What type of exercise or activity? Recommendation: The type of activity or exercise performed (running, swimming).