Maximum practice time recommended.

Bobbix

Is an hour the upper limit, or can I practice more than that?

HuduVudu

More is fine but make sure that you don't strain your voice. Also remember it is consistency that will build a strong voice. Try to make your practices the same length over a long time period.

As an example let's say you want to do 12 hours a week, make sure that you can do that for 6 or so months. Also if you are straining and you need to back off the amount of time that you practicing. Then make the shorter amount of time your new practice time. Don't just shorten the time and then a week later jump back up to the amount of time that caused you to strain in the first place. Again, consistency is key.

highmtn

If you have the time, and move along at a reasonable pace, without singing too loudly, you can practice for hours each day. Just make sure you don't oversing or strain. If your voice is getting tired, that's too much.