I agree. The Denso part number for the OEM coil pack on my '99 V6 is 673-1201. Google it and you can find it lots of places for about $65, including RockAuto or Amazon. Beats $100 list at the dealer for the exact same thing.
I would go with the cheaper of the two. I personally dont get attached to the toyota oem parts. I bought a oem coil from the dealer out of desperation and had to pay $120.00.. Ouch!
No official interval. I think most folks just wait until there is trouble--misfire codes that follow when the coil is moved, for example. I would bet that three-fourths of them outlast the rest of the truck.
I had one go bad and ended up replacing all of them with OEM from Toyota of Dallas. Their site sells them for $74. While I was in there I also put in new plugs and wires.