This is a very good topic... returning stuff in Jpn.
It can be hairy. Returning items has traditionally not been a part of Jpnese culture. Until ~10ys ago, I didn't even know that you could return stuff in Jpn. I've returned things in Jpn, but only things that are unused/untouched. I don't know if I'll ever be able to get used to this culture of difficulty returning things. I'm hoping that Costco will rub off on the rest of the retail industry in Jpn and eventually things will change.

If your item is obviously defected, or you haven't opened it or used it, you're usually okay.
But it's never like US where it's no-questions-asked.
The time frame for return varies by store, and it can vary also depending on whether the return reason is defect or otherwise.
Usually anywhere from a week to 30 days.
Some stores will not let you return certain types of merchandise (like food) under any circumstances.
Some stores require that you first call and notify the store and get prior permission before showing up to return.