I've been at this for years, and my income varies. With that said, I will say that I have more than one website. I have a larger niche site in the internet marketing niche and some smaller sites in a niche that I'm not going to mention. Sometimes, my IM site makes more, and sometimes my smaller niche sites make more. When it comes to which does better, it can depend on several things (like if I'm participating in an IM launch or the time of year - can be a niche thing).
As far as how long it took (or takes - building sites all the time) for my sites to generate income, I think it took until my third product review on my larger niche site (the IM one) before I started making money. I think it took like 3 months to get to that one way back when because I mostly posted articles to get more content on my site when it got going instead of focusing on the money.
As for my smaller niche sites, I have seen results in as few as 12 days -- even without having an e-mail list and just relying on the search engines (takes a few days for a new site to get ranked and start getting some traffic). I'm trying to get this time frame down to a week just for kicks. Since I have experience, I like to see sales within a month after I have launched a new site. Sometimes, it takes longer. I've had one or two sites that have never made any money. I let those go when the domains expire.
Joey