depends. if we are talking individual ideas, I went through 4 before I found something that was actually profitable(~$2-3K of affiliate marketing). 5, before I found that "million dollar idea"(it's not making 6 figures/mo yet...but it'll get there) if we are talking individual projects that number is closer to 50-60(since I made like 40 2-3 page made for adsense/affiliate marketing sites and 10 Q&A sites)
so, just to be perfectly clear, your success is in spamming - right?
those tend to die very quickly and generated either $0 or maybe $1-5 every 2 months.
the ones that actually worked were those that were actually useful to the people using them...so they linked it on other sites and kept it relevant.
i.e. I have a site that lists all available exhausts(even those I'd get no affiliate revenue from) for the nissan 370z...it shows the pictures, the specs, and the soundclips which allows them to make the right decision.
and it has an exact match domain - http://370zexhaust.com/
so I rank #1 in Google for 370Z Exhaust, and rank highly on pretty much any combination of BRAND 370Z Exhaust
so even though the target market is very small(that site gets less than 1K hits a month)...it's very targeted, so I get 3-4 sales a month(and that number will go up as more and more people get the car).
And since each sale tends to be in the $1.5-2K range...that one site brings in $500-600 a month.