This is not difficult!

Let's make it as simple as possible for those of you conflating the two things:
1. A visa lets you into a country within X days.
2. A period of stay lets you stay there for Y days.

The two numbers bear no relationship to each other in and of themselves.

If the visa is valid for 90 days you have 90 days to enter XYZ country. If it grants 3 years of stay then when & if you enter within 90 days you will (in the normal course of events) be granted 3 years of stay or status of residence or whatever you want to call it.

Holycow, it's as if some of you have never bothered to actually look at what a visa is and how they work. It's that sort of sloppy language use that gets space vehicles hitting local planets at a high rate of knots.