I'm not sure exactly when it's ending. You might want to keep an eye on the official announcements from the publisher or the manga creator's social media.
The ending of Girlfriend Girlfriend manga is quite complex and bittersweet. The characters go through various challenges and their relationships evolve in unexpected ways.
Renting a girlfriend in manga can happen in different ways. It could be a character responding to an advertisement, or getting involved in a strange pact. The details and mechanics usually depend on the unique world and story that the manga builds.