Great question. This is highly dependant on many factors including budget, traffic source, how helpful your affiliate manager is, how much research is being done with spy tools or other available resources.
Everyone is different and some people can probably get profitable after only 3 or 4 attemps, while others may take 20 - 30 attemps before finding something that sticks.
For paid traffic my advice is focus on one traffic type and the top proven niche categories...(spy and see whats currently running for a long time).
Try to master it and eventually you will know more about what will work and what won't work.