Honestly, I think film fifteen would be a good time to wrap the series up and give it a memorable sendoff.
Perhaps it could involve a few returning characters, the stakes could be closer to that of the original film, perhaps a few subtle call backs to the previous films.
Not sure what the plot would be, but I could see it ending with the gang finally growing up and maybe even book-end the films by having the final shot of LBT15 mirror that of the original.