Is a long shower the main cause of this leak? Would clear silicone on all grout lines help?
This could be a typical problem related to installing tile directly onto either regular drywall or the type called green board.
A far better installation is to install tile onto cement board and even better is to coat the cement board with a paint on sealer such as RedGuard before hanging the tiles.
One last thing is that the cement board really needs to be installed after the shower base is pushed up against the walls so that the cement board can come down past the turned up lips on the wall sides of the base.
Based upon this you should have a discussion about how the tile was originally installed. It  may very well have been done at less than current best practices in the industry to save building costs, especially if this house was built in a large tract or building project in a mass produced way.