Twenty or thirty or more years ago, I wrote to a cousin who DID know me and did not respond. About 5 years ago she died, her son found the letter and contacted me. 

I would say 1 out of 10 is great odds. Mine were not that good. 
Not everyone in the family is interested in the family tree.
A bigger annoyance: cousins who are working on the same family and will not answer emails or work with you! I have better luck with complete strangers.