There are limits, nothing is that easy. One webmaster can tell you it took 2, 6, 12 months to get US$500 per month on a project, but that doesn't mean is that easy, getting involved on building more and more projects so 4 x 12 = 4 years having 4 projects with US$500 per month = US$ 2,000 per month! no no.
Each project is different and if you read between the lines here at WebmasterWorld there are some who tried to replicate their success formula with other sites of theirs with no luck at all.
There was a contest (private contest) here at WebmasterWorld trying to get one site to earn X amount of money per month/year having only like 24 hours to develop it, something like that. Too bad we don't know the results :(
The first comment sounds kinda aggressive but Expected Man is right, invest your time in creating content, ideas, projects, even so curiosity is always welcome, in fact is the key to learn.
PD. The tricky thing is, some might say whatever about money and projects and there is no way to know for sure. I know some here are really on top of the hill in that sense, but they also manifest a great love for what they do (more than loving the money).
That is a hard question, some niches will get there faster than others. The variables are vast, my advise is don't focus on one site, learn how to find areas/niches that have the possibility of producing good results with less effort.
I've just sold one of my high earning sites and it was earning in the mid £1,000s a month. I've been pondering the question of how to do exactly what you ask about - an initial $500 a month.
After a few weeks of thought here are my conclusions, but treat them with a pinch of salt, every situation is different.
First, first and first, Devote your time to a subject you enjoy. There is no doubt about it, unless you are a very sad individual, always write about a subject you enjoy. Your enthusiasm will shine through and inspire your readers.
Next, as well as writing about a subject you enjoy, concentrate on a subject area which involves the purchase of something tangible. That way you can sell the "something tangible".
Thinking more, that's really about it. Nothing more. Except (!), spend some time and money taking pics and videos about your chosen subject - it will easily distinguish your site from the 100s of others which are publishing copied content solely to earn money off the back of others.
I have a site that I started about 4 years ago as a side project. I built it, promoted it, and left it on auto-pilot for the past 3 years. It is making around $400/month starting this year.
I think I could have got to that number faster if I didn't just leave it on auto-pilot all these years. Who knows.
But it definitely takes time. Time to build the site, time to build the trust, time to market, time to gain links, time to gain traffic... It all adds up to, in my opinion, at least a year or two to reach that level (for content site).
If you are talking about selling products/affiliate marketing, then the metric is different. You may not care about links, rankings, traffic. You may just care about finding that one killer product and landing page combo, and buying the right ads from the right sites... then you can easily hit that mark much faster. But you need to have some pretty solid marketing experience for that route.
That's using no tricks, not a throw away domain, completely white hat site, natural linking... all the stuff that slows you down but lets you get a good night's rest.
That is worth quoting. There is a forum I know where most of the members are bloggers, they share their stories and the majority (at least there) risk their accounts and domains and then they wonder why their sites and accounts were banned.
The most important thing in my opinion is building something GOOD that last, and generating good relationships with other people and webmasters, not something risky. Sadly, there are some young folks who wont listen!
I know a lot of people say just focus on good content and a subject you care about, but if your main goal is to make money with adsense, a lot also depends on whether your subject is one that will attract people who will click on the ads. It's obviously a lot easier to work on a site if the subject that generates clicks is also one you care about. But one site can generate $500 a month, another with the same traffic and same amount and quality of content, but a different subject, may generate only a small fraction of that.