An easy way to say would be when it's still 50% original, but I think an interesting way to look at it is that it becomes a whole new thing after every major change.
First it's his new car, then it becomes his new car with new tires, and then his car with new windshield wipers, and finally his old car with all new parts and some old ones. None of them are the same car.
I think in cases where it' a major rebuild, like turning a WW2 Minesweeper first into a ferry, and finally into Cousteau's research ship Calypso this outlook is more obvious. Are these ships all the same despite getting almost a full refit at each stage? I would say none of them are the same ship, but completely separate "things" with some old and some new parts.
This kind of thing is repeated often, but I don't think it's true. For one thing, how would tattoos last so long then?
More relevantly, I don't think neurons are replaced. There must be some material churn in the atoms and molecules that make them up, but even then different for different molecules - e.g. I don't know how much of our DNA molecules get replaced over a lifespan from the repair or other mechanisms.
The "on average" is doing an awful lot of work. Some cells are never replaced, some organs are replaced every few years or even partially over decades, some organs are replaced every few months (one of which is the skin).
Tattoos however, IIUC, sort of "float" between cells, and as those cells are replaced one-by-one the ink is kept in place by the surrounding cells that are still there.
