Does an informal relationship grant inheritance rights over the years?

There is no such period. Polish law has no acquisition of inheritance rights through living together: twenty years of a relationship give exactly the same rights to an estate as one year, namely none. Without a will the estate goes to the deceased's family under the statutory order, and the partner is not part of it.

Where the “after five years” myth comes from

The question about a number of years keeps coming back because in other areas of law time really does produce effects: adverse possession of property, limitation of claims, length of service. Statutory succession has no such mechanism. The circle of heirs is set by kinship and marriage, not by how long you lived together. Polish law also has no institution of a “de facto marriage” that would come into being on its own after some time.

What happens if you do nothing

The estate goes to the deceased’s family: children, and in their absence parents and siblings. For the partner that usually means several things at once: becoming a co-occupant alongside the heirs in a flat that formally belonged to the deceased, losing the power of attorney to a jointly used account, because it expires on death, and watching things bought together but “in the deceased’s name” fall into the estate.

One exception works in the partner’s favour: a person who actually lived with the deceased tenant succeeds to the tenancy. That concerns tenancy, however, not ownership.

What actually protects a partner

• A will appointing the partner as heir. In informal relationships the notarial form is worth considering, because a will in favour of someone outside the family is a classic target for challenges.
• A life insurance policy with the partner as beneficiary. The benefit does not fall into the estate and reaches the named person outside succession proceedings.
• Co-ownership of things bought together, instead of registering everything to one person.
• A joint account, which as a rule is not blocked after the death of a co-holder.

Two things have to be calculated in advance: the reserved share due to the deceased’s children, spouse or parents, and the tax, because an informal partner falls into the third tax group. Both have to be counted before the will is signed, not after.