I don't know if there is an exact time for the last reservation (that would seem to depend on the closing time each night), but the TS restaurants do close earlier than the park by at least 2 hours or more. The kitchens seem to close around 9pm or 10pm, no matter what time the park closes. You can ask the Dine Line for more specific info on this.
We're local, so we use the Dine Line for all our reservations. I find it easier to be able to speak with a human being when making reservations.