The above post from webkew says that you can build a site that gives 3000 visitors per day. If it is a good site and if each visitor visits 4 to 5 pages, 15,000 page views a day is the possible number.
This also implies 100,000 visitors per month and that will make you in the top 100,000 sites on alexa.
This may give you (if you make $5 cpm) $2000 per month. The article says you can make a page per day with 2 hours per day effort, and therefore with 730 hours of effort per year you can make $24,000 per year or $32 per hour.
Is it easy? No, very difficult. Is it worth trying? Yes
What type of skills are needed for it?
Give your ideas