Facebook PVA Reverification?

Can somebody please explain this process to me in depth? How long does it usually take for there to be a reverification? Is there always a reverification? How does it work? For some reason, when I add a phone to an account, I don't have deal with reverification, but I have heard that it is an issue.

I would really appreciate if somebody could help me out here.

Thanks

when u add a phone it can ask u later to reverify it, if so u need the same number u used before

time not accurate can be 1 day months or never