Any laminated decal for the most part should last easily 3 years at minimum with the cheaper cal material, typically 5 from what i have seen with the 210 and 3M 8509 laminates. Un-laminated i know the Roland inks are rated 2 years outdoors but it will fade some. I'm sure the Mutoh inks aren't far off.
Depending on the use of the decals i will laminate them.
If it's anything going on vehicle paint or windows i laminate. If it's something that will be like a indoor label, yard sign or something i tend not to laminate it.
The size and shape of the graphic also depends on if you will laminate or not. Laminating something that's large will help with application vs no laminate which is alot thinner.
I typically do laminate everything for the most part though. I used to use oracal 210 and now use 3M 8509 and it makes for a overall nicer looking finished product and resists the inks further from fading or being scratched up.
I do bulk promotional decals for automotive companies too and i laminate them because people will be throwing these on toolboxes, car windows, intake manifolds and the whole nine so mine as well do it to fit all needs since laminate isn't much more cost and it's the right way to do it pretty much.