I am sorry to hear this, and I agree that fencing the line is appropriate.
PA Code Title 42 Section 5530 gives you 21 years:
"§ 5530. Twenty-one year limitation.
(a) General rule.--The following actions and proceedings must be commenced within 21 years:
(1) An action for the possession of real property
(2) An action for the payment of any ground rent, annuity or other charge upon real property, or any part or portion thereof. If this paragraph shall operate to bar any payment of such a rent, annuity or charge, the rent, annuity or charge to which the payment relates shall be extinguished and no further action may be commenced with respect to subsequent payments.
(3) (Deleted by amendment).
(b) Entry upon land.--No entry upon real property shall toll the running of the period of limitation specified in subsection (a)(1), unless a possessory action shall be commenced therefor within one year after entry. Such an entry and commencement of a possessory action, without recovery therein, shall not toll the running of such period of limitation in respect of another possessory action, unless such other possessory action is commenced within one year after the termination of the first."
I hope this information is helpful.