How often should a dancer practice?

That depends. You can practice a few nights a week for a few hours per session, or you can practice 5-7 days a week in 30-60 minute sessions. It's better to practice 5-7 days a week if you're trying to memorize a step. 3-4 nights a week is good if you're busy, but need the exercise. 

It's totally up to you. Try to take some time to discern what practicing schedule works best for you. I personally find that I can do about 5 days per week and I do 30-45 minute sessions. It gives me the time to work on my posture, stage presence, and work on my solos (I just realized that "solos" is a palindrome).

Make sure that you're not overworking yourself. If you exhaust yourself, you may end up too tired to practice the next day. 

Here's my practice routine:

I get up in the morning, stretch/warm up, practice working on my posture, work on new steps, and then cool down.

I complete my schoolwork, warm up again, work on my steps, then cool down.

My first session is about 45 mins to an hour long. My second session is around 20-30 minutes.

I do this on Tuesday, Thursday, Friday, Saturday, and Sunday. On Monday and Tuesday, I have a 45 minute dance class. The first is my core class where we learn dance steps, the second is my show class where we learn to perform for people.

Be patient with yourself. Don't get frustrated with yourself if you don't get a step right the first time. People learn at their own paces. If you're a slow learner, that's fine. There's nothing wrong with that. I happen to be a perfectionist, so I work... and work... and work... and work hours and hours on end until I finally get the step close to the way I want it. Then I'll go back and work on it again. If this is the route you feel you need to take, that's fine.