I have one of these businesses. Took me 2 months to build the product (on the side, after my full-time job). A few more to market and see sales (it takes time to close sales and generate site traffic). It should generate 10-20k this year, hopefully more next.
My advice is to focus on a niche market filled with people w/ money, willing to pay for stuff. And avoid any market/problem that your typical computer geek would be interested in or aware of (you want to relax on the beach, not code away 60hrs a week fearing some other startup will catch up and take the market away).
I think people are overly concerned about competitors.
At least two people now have read my blog and set up their own businesses. One of them practically Xeroxed my business, and his I-can't-believe-its-not-me is fairly decent. I always joke that since my sales have doubled since then all I need is to recruit two more people who understand copy/paste and then I can retire. (The numbers in the joke are way out of date.)
Similarly, I was myself a new upstart competitor to the established market participants back in 2006, and I haven't killed them, either. (The idea is not to pick a market with no competitition -- that is a red flag there is no money to be made. Instead, pick a market where there is an addressable niche for whom the existing choices are painfully inadequate. Then, sell pain relief.)
My advice is to focus on a niche market filled with people w/ money, willing to pay for stuff. And avoid any market/problem that your typical computer geek would be interested in or aware of (you want to relax on the beach, not code away 60hrs a week fearing some other startup will catch up and take the market away).