Earlier quoted context omitted.
From what I’ve gathered, Japanese whaling is about as traditional as British rule of Africa, India, and America. Although the article quotes a hunter claiming their village has done it for 400 years, it also later says “A number of coastal communities in Japan have hunted whales for centuries but consumption only became widespread after World War Two when other food was scarce.” Also, whales aren’t being farmed, they…
What is the correct amount of time for a tradition to exist before it deserves to be protected?
It's a question of whether argument-from-tradition is a valid argument for continuing a practice. For relatively new "traditions", it's not. You need other reasons.