On our Honda Odyssey, the manual says every 8 hours (clock hours, NOT towing hours). Usually, I will redo the procedure roughly every 3-4 hours while travelling. This usually corresponds with an "eating break/fuel refill/stretch the legs" stop. The tranny in our Odyssey has a known weakness due to lack of lubrication in certain circumstances (not towing related). I don't want to take any chances so I do the procedure more frequently than needed. It doesn't really cost anything and doesn't hurt to make sure everything is well lubed.
Not sure about your manual, but our manual also recommends that the tranny fluid be changed on an accelerated scheduled if the vehicle is flat-towed. This is because the odometer doesn't register mileage while being towed but the fluid is still "in service" while towing. So I usually change ours once a year which is overkill, but as I said, the transmission in our Odyssey had known problems. We, fortunately, haven't experienced any problems with it and I'd kinda like to think it was because of the extra frequency of the pre-tow procedures and frequent fluid changes.
MAKE SURE that you do the pre-tow procedure properly. If your pre-tow procedure requires you to shift down through the gears first, then back up with the last shift being from D to N, make sure you DO NOT shift it into R accidentally. If you do, you need to repeat the procedure again starting at the Park position or else transmission damage will likely occur.
~Rick
