You could add wood shingles to your project and let the mycelium grow into the bottom of the shingles from below, and give it a water proof foundation up off the ground (perhaps on a pile of rocks or something). This coudl extend the outdoor life time a good bit!
There are a lot of variables, so it’s hard to say exactly how long the sculptures will last. The size of the sculpture will matter quite a lot, as well as if you decide to put a coating on it or leave it all natural. I don’t know what the weather is like in Norway but if the sculpture has time to dry out (in between rainy days) then it will last much longer than it would in constant rain. Have you seen Timothy Hull’s _The Accelerated Ruin_ project? I think you’d find it interesting.
I have read one month somewhere. for small things but it clearly depends on the size. of your structure and if you can make it rot in stages b growing around boxes that keep the mushrooms dry. so it slowly falls appart but that what falls apart could be a art work of it self. for instance you could put a bag of seeds wit flower and ground in the structure and when the thing rots. it will will go open and from the ground you can get flowers that attract bees that can also make use of the structure. stuff like that seems allot of fun (: