When you do a lousy yoga class, the type that can injure you. You know about it because your agitated body tells you immediately.
You can find a class that works for you. But the best practice is your practice, where the breath is recognized and with movement that is best for your body today. Because your body and intention may be different from yesterday.
That short, medium, or long length of time at practice is yours and with the breath, your practice is never lousy and always worthwhile. Follow the breath softly and slowly, extend the exhalation and feel the joy.