I've had some niches turn a profit in two days and others take two weeks. Then I've had some that are just worthless. The key is to test with out doing a massive amount of work. Pick two or three niches at one time do some redirects to affiliate pages. They are eventually going to get to the products sales page anyway so why not send them straight there to begin with and see how it converts.
You can do this with articles, building rss feeds from those articles, dropping answers at yahoo answers and wikki answers. Once you know if it converts or not then start building your supporting pages and a landing page.
As far as when do I know a product is a dump? If I go about 600 hops and I don't see a sale, then I'm not promoting it any more. But up until that point I haven't done anything but the above to it so it's ok. You never know, because some times you may get lucky and still get a sale out of it later down the road. Because the content you already put up is still going to be there.
Some niches you won't get 1 sale out of 200 hops and then the next day that same niche will get you 3 sales out of 75 hops. It just all depends on who's in the mood to pull out their card that day.