Use io.ReadFull to slurp up the desired number of bytes on each iteration of the loop.

buf := make([]byte, 10)
for {
    _, err := io.ReadFull(r.Body, buf)
    if err == io.ErrUnexpectedEOF || err == io.EOF {
        // Success!
        break
    } else if err != nil {
        // Something bad happened.
        log.Fatal(err)
    }
    time.Sleep(time.Second)
}