I meter at 25 and let sit for a solid minute just for kicks. It's somewhat temperature dependent and I prefer being safe to sorry.
I think Vaughn is right when he says developing for longer than the recommended time probably doesn't do much because of the reasons stated. In fact, it may reduce density in the negative and make the positive darker, as it could allow more silver to move from the negative to the positive.
All please genuflect... a reading from book of St. Ansel
Polaroid Land Photography, pg. 228.
"Effects of development changes: Essentially none, since the print and negative develop to completion. A very slight decrease of the base-plus-fog density of the negative occured at 60 second. Insufficient development must be avoided, since it produces discoloraton of the negative when it is exposed to light."
"Tested optimum development time: 30 seconds at 70 deg. F"