50_dollar_bag
Imitation Louis Vuitton
hammer decay starts after 50 turns for buildings, and never starts for wonders.
Hammers apparently do decay for National Wonders, I tried this last night on an Emporer/Marathon Fractal map as Huyana Capac. I'd cleaned out my continent and was quite far behind in techs because my trading partners were Tokugawa

Teching took a while to gain momentum so with no need to build military and my core cities up to date with buildings I gave this a go. I built the Hermitage about 5 times over and got between 750-800 gold for each failed attempt, i did notice that a few of the cities which had built it to 1 turn to completion eventually showed 2 turns after about 100 turns, so decay does occur.
It did save my ass from a sneak attack from Monty as I let the Hermitage complete and the refund gave my enough money to upgrade some troops, about 5 times more than a turn of 0% science.