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You can use your voice as much as you like, as long as you don't overuse it by singing loud. You may not yet have the stamina to sing all day, even lightly, but if you keep it light, use cord closure to avoid putting too much air on the cords, and keep the pressure down, you could sing at a lower volume just about as much as you feel comfortable doing.
You can't push your voice or strain it for hours a day. You shouldn't strain it at all.
Light, sensible, exercise for your voice is fine. Some singers work their way up to singing exercises for several hours a day, after warming up. That is serious practice, but will develop your voice more rapidly.
The voice includes many muscles, and no part of your body grows overnight or even in a month to a significant degree. Take it steadily and reasonably. If you have the time to sing more, then do so properly and with care for your voice.
The lip rolls get easier as you continue to do them.
Many students have difficulty doing them at all. I was one of those who couldn't get them at all. It took me a while to relax my face enough to do scales while trilling the lips.