Preserving your windowbox
Being made from wood, your windowbox is clearly not going to last forever, but you can prolong it's life by giving the inside and out a good couple of coats of wood stain after assembly, which will also disguise any faults in your wood (and to a certain extent any splits that came about from trying to get the planks in the first place!).
You can also of course choose the woodstain colour to fit in with that of your house to make it a more attractive feature (and not looking quite so much like dismantled and reassembled pallet wood!).
It's worth repainting the outside of the box on a yearly basis to keep it looking good, although inevitably the windowbox will rot from the inside out over several years which you can't do too much about.