How much time building the site vs promoting it

Hi,
I guess I do it pretty much wrong in this matter so I would appreciate to hear your opinions about my approach and how you do it.

I have a Multimedia Design background and can do everything for a new site on my own. The graphics, php, javascript and whatever. I see myself building up project after project and spending most of the time on building the site and the functionality. After that I promote it a bit and then jump on the next project.

For example my last project cost me 4 weeks of scripting, designing and so on. After the launch in August which gave me 6000 visitors the average amount of visitors a day is now around 600 and 1300 pageviews. I had great positions in Google for the first month and then I guess got sandboxed and now only rank well for long tail keywords. I plan now to let it sit for a while because it is somehow a reference site which has timeless content and will come back over time. It also has my main key phrase as url and even though the toolbar doesnt show me a pr backlink checks say it has a pr of 6. I earn around 2-3$ with Adsense from it every day.It is not full of Adsense or affiliate links because I want to establish some credibility first, before I try to get more money out of it.

1. Do you see any flaws in my approach?

2. Do you think it makes sense to focus on promoting the site instead of letting it sit and repeat the whole thing for some other niches in which I know it would work?

My idea is to try to repeat this as soon as possible for other niches that I have these projects sitting there until they come out of the sandbox. Then I can focus on pushing each individually instead of promoting one site right now but let other people take these spots in similar niches.

3. How much time do you spend on building the site vs promoting it?
In my case it is 70% building the site vs 30% promoting it

4. What stops you from jumping on the next project?
In my case nothing stops me and I guess that might be a problem.

5. What kind of projects do you think can generate immediate money?
I need some more money right now even though it makes of course more sense to build sites that will be there forever. I thought about doing some information projects... but can't think about anything else except going back to what I did before and doing web design.