For voice students, a lot depends on where in their development they are starting, their overall health, and what level of repertoire they are learning. Most beginners should do 15-30 minutes a day, consisting of vocalises (vocal warm-ups) and work on songs. If a student is not feeling well, especially if they do not have a solid technique, I strongly recommend waiting until they feel better before practicing. Without a solid technique, they can cause some damage and develop poor singing habits. Having said that, the more frequent the practice, the faster the progress. Also, voice students need to spend time listening to good singing. Since most singers on the radio do not sing with a healthy technique, I will often provide suggestions for them to listen to, and listening should be a part of their practice time as well.