Hi Elly, 18 days is just too many days to be with a stranger roomie.
Most likely the only person who would be willing to cruise that many days is an older senior. They have the time and the money. Plus, they may have some medical problems at that age I am just guessing.
There are some younger singles in their 40's on up who are looking at the longer cruises. Good luck on your search for a roomie. Angelgal