How often you practice, and for what duration, depends on your goals as a singer.  We all know that practicing improves skill, but do you want to become the next opera singer, a pop star, or do you just want to improve your singing-in-the-shower skills?  For dedicated singers with professional level goals, or professionals preparing for concerts, you might practice as long as one or even two hours a day.  On the other end of the spectrum, even just a few minutes of practicing a day, or 15-30 minutes two or three days a week can pay off in big ways.