Hey Lenny....just like conversion ratios, rebilling percentage takes a decent amount of signups to get an accurate feel. The reason is, there are even more variables in rebilling percentages than there are initial signup ratios...
The first and foremost indicator (besides the sponsor having a shit site) will always be your traffic source. TGPer's will never rebill as high as search engine traffic gurus, and even search engine gurus tend to rebill less than people with good opt-in email lists. Other considerations are what processor is being used, does the guy have enough on his card to allow billing in the first place, etc..
I would hazard a guess that it takes at least 50 sales to get a valid idea of what your retention is going to be like, as long as you're in reasonable norms to begin with. If you go 1:10, then 2:20, etc...I'd say you're getting your answer pretty quick.
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