It depends what you call successful. Getting practical programming experience, then yes 90% are successful. Code amounting to something useful however would be hovering around 10% or lower.
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It's a bit much to expect continued maintenance far beyond the agreed period of a summer, isn't it?
If the code is merged to the project, it's usually the project's responsibility from that point on.
To expect? GSoC participants are students. That means for most of the year they're busy doing school work. You expect students to be on the hook for continued development of their summer work?
Where's your code, anyway? What free software projects are you working on that we should expect to see progress on?