How can I sing for longer periods of time without losing my voice?

Question:

How can I sing for longer periods of time without losing my voice?

I'm a pretty powerful singer (I can sing loudly), but after about 2-3 hours of singing I have almost no voice. I've studied voice for 4 years. How can I keep my voice strong?

Additional Details

7 months ago
Thanks for the responses. Maybe over 3 hours is too much.. everyone takes a different amount of time to warm up. I will try to keep these things in mind in the upcoming weeks.

Best Answer - Chosen by Asker: 7 months ago
Thanks for the responses. Maybe over 3 hours is too much.. everyone takes a different amount of time to warm up. I will try to keep these things in mind in the upcoming weeks.