Truthfully? There is no good explanation for not having a 2, 4, or 6 hour change. Most fights don't last more than 2 hours and most furballs happen away from the base-takes (which is why the bases don't get defended well enough to kill the capture attempt).
One augmentation on Grizz's suggestion might be to examine the prime time vs off peak and have side switching become variable based on times of the day. Off peak may change every two hours and peak time may change every 4.
Either way, 12 hours is ridiculous.