There is no way that you can just assume ownership of an encroached strip of land unless 1. You've had possession for over 10 years AND a title to the encroached land was registered. Even then the Registers of Scotland are unlikely to accept such a title unless no owner can be found so the answer to your question is that there is no timescale for prescriptive ownership without a title to the land in question. That hasn't happened here so if your mother's house is encroaching the options are 1. To reinstate the correct boundary or 2. To buy the strip from the neighbour and have both titles adjusted at the Land Register. It is usually cheaper to reinstate. I'm afraid these are the options if the neighbour is insisting on this being regularised. Happy to discuss further. I hope that answers your question. Please leave a positive rating so that I am credited for my time.