How many hours a day can I safely practice singing?

Question:

How many hours a day can I safely practice singing?

I have studied privately for a year, so I have a reasonable foundation. Assume I constantly move through the whole *comfortable* range of the voice. I am trying to develop a beautiful, natural, tension-free sound, and improve my intonation. I am willing to work as much as possible at this because it is a career for me. I'm currently striving for about 3 hours a day, which consists of mainly scales, arpeggios and long tones on a wide variety of vowels. I've done this for about 3 weeks now and have noticed some improvements. My voice does feel tired at the end of the day, but there's no pain or hoarseness. If I can practice more, I would like to. Also, any suggestions on what to concentrate on when I practice? What to think about, and what exercises to do? I want a tension-free sound with a nice vibrato (it only creeps in once in awhile for now). I will resume singing lessons in a few weeks.
No facetious answers please. I think only a professional singer or singing teacher would know.

Best Answer - Chosen by Asker: Not a lot of pro singers or teachers here, so I'll help as much as I can.

The main thing you want to avoid is the pain or hoarseness you mention. It doesn't help to increase the time in one day in which you practice, just make sure you practice consistently. Singing is mostly about developing muscle memory, and that takes time with mental processing time (e.g., sleep) in between lessons.

In order to develop a truly versatile voice, be sure to keep changing your practice routine. New material will help you with interval jumps, range extension, transitions with throat voice or falsetto, vibrato, vowel formation, and more. By repeating the same exercise or song, you only get good at that song.

Recording your voice and playing it back may give you additional ideas of things to do, because our perception of what we sound like is influenced by the vibration of our body, and what is transmitted through the air is different.

Working with a tuner may help you to make your pitches more exact and consistent. Working with a piano or keyboard may improve your pitch recall. Try to focus the sound lower in your chest to achieve a more solid note and consistent vibrato.