I do not know of a workaround to extend the speaking pause time before it asks you if you would like to send it but there is another way to essentially achieve the same functionality of "writing" or "speaking" a text message should I say. From your messaging app tap on the bar as if you were to send a message and the keyboard will come up. There should be a little option down at the bottom left hand or right hand corner to use your voice to type. If a microphone icon isn't there, try holding down on the little gear and more options should come up. This applies if you are using the default keyboard and not some third party one, although there are exceptions. If you have any questions, please don't hesitate to reply.