Is a wood shed with 12mm walls worth it?
After further contemplation I suspect that we're discussing siding, not sheathing. The latter is installed over framing and under siding. Siding is probably plywood or pressed fiber hardboard.

Most sheds (and indeed most homes) in my region are built using 7/16" (about 11mm) oriented strand board (OSB) wall sheathing underneath some sort of siding. This is standard. Wood panel siding is rarely used anymore except on sheds, but 1/2" (about 12mm) siding panels of good quality provide plenty of strength and durability when applied over good framing. 

How long it lasts depends on now it's cared for and protected. It should be primed and painted or otherwise sealed initially and every few years thereafter. Ideally all edges are sealed, particularly the bottom where rain splash will soak into the wood, causing swelling, decomposition, and eventually rot. Plywood or hardboard siding that's cared for and doesn't reside in perpetually wet locations can last 10 years or more.