Lisbon is one of the very few ports where most of its departing cruises are one-way options. The most common endpoints are Barcelona, Rome, Athens, and London.
For the most part, the choice of endpoint determines much of the itinerary. For example, ending in London usually means a cruise that visits the western & northern coasts of Spain and France while one that ends in Rome visits the eastern and southern sides of those same countries.
Most of these options are in the 7 to 14 night range, though there are some 3+ week cruises as well.
Trans-Atlantic repositioning cruises are relatively common from Lisbon and can make up over 10% of all departures in a given year.