It will shrink within a month or two depending on the climate. The shrinkage won't impact the longevity just the gap at the joints, I think 10 years is the most you could expect out of any fence.

I have some 20 x 120 x 3000 larch wood planks and want to make privacy fence at my small summerhouse - three meter long, two meter high (15 planks with 30 mm gaps). What did you use for the vertical beam structure to bear the horizontal planks? And, it looks like the whole thing is set on a concrete basement, right? I guess, I don't need to know how the gap joints are made since the fence is only as long as the planks are. But I'm still interested to know, maybe later I'll do a lower fence for the garden. I will also seal all the wood - I live in Russia, it's +35 in summer, -35 in winter and much rain in between. I hope the larch wood will withstand it at least for few years. Thank you.