The calendar on Shawn's desk says that it is February. However as Shawn and Gus are driving to Mary's the leaves have changed colors and are falling which implies that it is fall.
After Lassiter and Henry enter the prop car, Henry states the car has no ignition, not even a place for a key. Moments before, there was a shot of the steering wheel and dash board. It clearly shows the car ignition.
During the opening flashback to 1989, Henry refers to the running time of The Little Mermaid being 116 minutes.
The running time for The Little Mermaid is about 85 minutes.
Since Shawn is really going to see Psycho, the running time for that is about 109 minutes
So Shawn would have time to see all of Psycho, but Henry seems to be way off.
However films do have commercials and previews, and they even didn't have tickets yet, so the fact that Shawn's father mentions he will be back in exactly 116 minutes would indicate that he took this into account.
However films do have commercials and previews, and they even didn't have tickets yet, so the fact that Shawn's father mentions he will be back in exactly 116 minutes would indicate that he took this into account.