There isn't a simple answer to that question. The amount of prebuffer is not a fixed amount, but depends on many factors, including the measured network bandwidth, recent history of rebuffers, the type of demux being used, the skew between video and audio in the stream, and other things.
I'm very unclear about what you're trying to do. Why are you making this measurement? Is it just curiosity? Or does it have some functional purpose? Is the "application" that you're writing a Roku channel, or something on an external server?
--Mark