I applied for a job today online, how long should I wait to call back?

It depends on which company you applied to and how busy they are at this moment with job applications and such.  

The earliest I've heard back was two days and the latest I've heard back is that they didn't get back to me at all. I think you should wait for three months at the very most. 

When they haven't gotten back to you then, it's most likely because they aren't interested in hiring you anymore. Most companies actually say this on their application I believe (or at least the places I've applied to said that).

This depends on the employer. Various employers will depend on many different ways of contacting each other and/or if you're worth hiring or not. Specially for summer jobs - they want people with experience and determination.

You can always do some volunteer work and put it on your resume. The more extracurricular activities you have, the higher your chances are of getting hired. And once you're hired, you'll be able to put work experiences on your resume, thereby improving it.

If you're looking for a teenager summer job though, it would be better to try going for something that you like doing. As with any job, actually. If you don't like what you do, you'll absolutely despise your job for the rest of your life.

Pick something wise. Don't get a certain job just for the money when you hate it, because doing that is not as easy as it sounds. In fact, that would be torturing yourself. For example, I do private math tutoring because I'm superb in mathematics and I love teaching it to others who need it.

Def do not wait... Telephone the company ask for HR/Personnel department and ask how your application is going, ask how many people applied for position ask if they need any more documentation ie copies of qualifications etc keep pestering them (not every day) weekly maybe. BUT do not wait it will look like your not bothered and companies want a reaction/commitment/dedication from their employees.