I know that this question depends on what kind of business one is in.
But if you where to get a healthy business out of a website based on advertising how many monthly users would you need to have, on average.
It's a tough, tough go.
Best of luck,
I think it's better to warn you: it's really hard to make any money with advertising online. Most likely, your earnings will be precisely $0.00 for a long time.
Thousands of page views for a generic site will translate into about $0.50 of income. Really. And to get those 1000 page views will take work.
It also depends on the revenue per customer, how good your conversion rates are (i.e. do 1% of your visitors become customers, 10%, 75%), and how much money you want to make.
If a single sale gets you $50 in profit, then at 1% conversion rate you would need approximately 10,000 visitors to earn $5,000 per month.