What I'd suggest for buying PPC traffic is buy a few day's worth, whatever your budget is, then stop and wait maybe 2-3 weeks. Then see if you amde a profit or not, if so buy more and keep it flowing, if not - well try something or someone else
It depends on the site of course, but I think 2-3 weeks is enough time to see. Some will still convert later on of course, some will convert faster. You have to try for yourself - unfortunately, that means being patient.