I've done a lot of testing with this...http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1280424364
I was trying to make an autoranging ammeter.
It only takes about 5.5 ms for the reference voltage to switch from DEFAULT to INTERNAL, this delay is because of the 100nF cap on the AREF pin. In your loop you will actually have to throwaway the first 50 readings or so before you get a stable reading.
Also keep in mind the stabilization time does not start from when you change analogreference, it starts from the first analogread after you change the reference.