I've received relocation assistance and signing bonuses where it was stated I would need to return if I left within a certain time (a year I think)

Can't speak for anywhere else, but it was clearly established at Amazon that any relocation assistance had to be returned (prorated) if I left before 1 year. I don't imagine that's terribly unusual, but many companies might not make it as obvious if they don't have the same high turnover rates as Amazon.

its not uncommon to put a time bound on it - like 6mo or a year. never heard of it being indefinite

That sounds like bad financial advice, and that you’d be be better to accept it and put it in savings for however many months. If you don’t stay the required amount of time, return the money. If you do, ta-da, it’s yours.

Ask your tax person first, of course.

If you accept relocation assistance and then don't relocate it, I'd expect the lawsuit to be open and shut.