According to the recommendations, you can speak up to 30 seconds at a time:
My new iMac desktop computer (2013) has a built-in Mac Speech Dictate feature. It is doing fairly well, albeit with some hilarious mistakes. While dictating an email, the icon changes message to "Done" while I am still talking. Then it prints what I have said. My question is why does it say done before I am done? Thank you.