Forever, without exaggeration. Sport fishing in Japan has existed long before cameras were available to record those special catches, so the peculiar artform known as gyotaku developed. As we all know, certain anglers like to exaggerate their achievements but a gyotaku provides irrefutable evidence of the size of each fish, as it is direct 1:1 impression of the fish. My friends from back home jokingly referred to gyotaku as ‘fish Turin shrouds’, which is only partly correct I guess as gyotaku are a print of the real thing! Anyway, the nice sole I took last month was over 40cm, which is the size over which the boathouse will make a gyotaku for you, for free (they actually take two prints, one for you to take home and one to put up on display in the window of their office). I then took mine to be framed, which not only preserves it and makes it look nice, but also the framers will flatten out the ruffles in the washi Japanese paper it is printed onto.
One nice thing is that the gyotaku lists not only the size of the fish and who caught it, but also the date, place and name of the boat it was taken on. Mine comes with the official seal of the boathouse, although it is more common to name the skipper or third party who confirmed the details of the catch at the time (this is known as gennin in Japanese). Also, the ink used to make the print is natural sumi calligraphy ink, so it washes off the fish with water and of course the fish can be consumed afterwards as normal.
Being made from traditional Japanese paper and printed with sumi ink, a gyotaku will last for years. I have seen several pre-War gyotaku, in various places, and even one from the Edo Period. I hope mine will last as long!