Is there a way to extend the time you can talk to Siri for?
You can prevent Siri from cutting you off in the command mode by holding down the Home button until you're done speaking.  

Normally you activate Siri in the command mode by holding the Home button until Siri responds, then releasing the button and talking.  In that mode she'll cut you off pretty quickly.  Continuing to hold the Home button bypasses that limitation.