As you may know, I have been a big fan of 30 minutes of exercise everyday. That how I started my fitness journey with that commitment.
A lot of big box exercise programs lean heavily into the time value for marketing to consumers. Programs like T25 and 21 day fix have a prescriptive course of exercises that are 30 minutes or less. They are definitely worth it.
Boutique studio classes are usually 45 minutes in length. This allows time to clear the studio before the next class as well time for a cool down. Spin and yoga studios are most likely to use this as a metric.
So Phyllis4fitness, whats your point?
Since I have taken the NASM PT course, I have learned that it there is no magic number. Its not the time its the program design that makes the difference, its the quality of the workout. If you are doing a chest day, you may need more than 30 minutes to work out all the muscle groups. If you are doing a leg day less than 30. Once your fitness improves, you can add reps, weights and length to your workout.
Caution- remember the power of the rest day. Rest is vital to muscle recovery and will accelerate your overall performance the next day you hit the mat.