All products that I make are fully treated for the first 18 months of their lifetime and are fine to be left outside all year round.
After 18 months, I would suggest that you treat products with a good quality woodstain made by Sikkens or Sadolin. You should then retreat every 2-3 years depending on the harshness of the weather.
If you choose to cover your bench or seat set with plastic sheeting over the winter months, you should make sure that air is still able to circulate around the item to prevent the wood from sweating.
With planters, I would highly recommend that you line them with polythene prior to planting them up.