Overuse can damage your vocal cords, and if you often find that you have lost your voice at the end of the day or after an hour of singing, your vocal cords may be suffering tissue damage. The vocal cords need a full 48 hours to recover from vocal fatigue. Anything more than 2 to 3 hours a day is too much. If you have excellent technique and sing regularly, your voice may feel fitter and be able to sing for longer periods of time without getting tired.
Any song with vocal effects such as distortion or glottal attacks (such as rock music) can take its toll on the voice more quickly than, for example, soft folk singing or light opera. That doesn't include time spent learning to sing at first sight, dictating, playing the piano, and absorbing knowledge related to singing, such as anatomy, music theory, and music history. Unfortunately (as is the case with so many questions about singing), it is very difficult to answer and it really depends on the individual, the style they are playing and what stage they are on. I assumed that if I did them by memory hour after hour, eventually, my body would convert them into muscle memory.
I also record everything I sing, so it's not like I'm literally devoting all my practice time to singing, when I actually listen a lot too. Constant practice will also allow your brain to quickly grasp the concepts associated with good singing. But didn't we just say that there is such a thing as practicing too much? True, and an amateur singer who belts in his car a tune that is out of reach for 2 hours is significantly different from that of a trained vocalist who practices for 2 hours. When you start out as a singer, or at least, you start to take it very seriously, you probably don't have the stamina to keep singing for long periods of time.
It will end up being a waste of time that was supposed to be dedicated to singing and it is likely that you have not progressed. For example, if you have never sung before and have just started vocal classes, doing 3 hours of practice will not benefit you: the muscles in your voice are not strong enough to hold up to 3 hours of exercise. If you are serious about your singing career and want to pursue it professionally, you should visit the Atlanta Institute of Music and Media.