When did Adam Sandler stop being funny?
The tough thing in gauging that question is to determine what Sandler was exactly. He was one of the most unique SNL cast members ever given that he wasn't particularly memorable in sketches. His specialty was Weekend Update, and he had characters that were memorable, but tended to be hit or miss with the audience. That lead to a movie career that was very similar in the early years. Several movies that were memorable but were hit or miss. If you were a kid or teenager when those movies came out, you probably find Billy Madison, Happy Gilmore, and The Wedding Singer to be very funny.
Those movies transitioned not so much to things that were product placement-filled, but celebrity cameo-filled. THAT is what I feel cost him the humor that he was bringing to the masses in the mid to late '90s.
He also had a thing in his later films about him being some chick magnet, whom all the women swoon over. He's also almost always rich, successful, living in a mansion or upper middle-class house with a hot wife, and he's even a badass punching people out with one punch.