You could workout safely 7 out of 7 days a week, but 100% isn’t required to achieve. If you’re trying to make up ground, 5 or 6 days works great. For those looking to steadily add to their accomplishments, 4 days is the perfect balance. For 3 workouts a week, you’re usually just trying to maintain health and keep the weight off. Any less and you should be having an off week. 2 or less won’t be enough to achieve but it’s certainly better than nothing.
If you fall within the 3-7 range, remember that your intensity is the biggest contributor to success. When you combine both consistency and intensity, you create an extremely powerful combination capable of churning huge gains.
I recommend aiming for all seven days a week so that if you can, you end up in the gym every day of the week. When stuff comes up (and it will), take the day off and chalk it up to rest. This should result in 5-6 workouts per week on average. Keep up the intensity and your results will go through the roof.