There is no statue of limiation, it actually works in reverse to that, After having had adverse possession of the property, in a open manner for more than 20 years, they may attempt to claim ownership. So they will have had to had that encroachment for over 20 years to make a claim.
And it is not automatic they have to file a suit to take possession of it.
Prior to that the legal owner may merely sue to reclaim their property
Adverse Possession laws - Information on the law about Adverse Possession - Requirements
