Knowing how your traffic performs in general is a big part of the answer to that question. Look at how well other sponsors have done for you, and decide on both a ratio as well as timeframe, and as soon as you reach one or the other, it is time to clean house.
Back several years ago before I had any idea what I was doing, I remember being 0:13,000 with an outfit called Spice Cash.
Now I rarely let a sponsor go past 0:4000 unless they have preformed well for me in the past, I might let them go to 6K, and usually if I don't see any results in a month I know it is time to find a new one.
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