This flat is perfect for both short and long rentals. We (3 students) have spent there 3 months and it is the best deal for its price - spacious and sunny flat, great location (close to touristic city centre, 2 metro stations within 3 mins, 2 supermarkets, many hipster cafes/restaurants. As most of the older flats in central Lisbon sometimes things broke down, but it is in great condition and has every furniture needed (including TV). The neighbours are nice and friendly too, never complained about noise or anything.
Maria is the most wonderful host you could hope for. She answers immediately and helps with everything (repairs, cleaning, changing bed linen, providing heaters and fans) and is the nicest landlord. I highly recommend her and this flat not only for tourists but for students as a great alternative to the renting platforms - there are 0 chances of finding something better for this price and Maria's flat has truly become a home for me and my flatmates.