We all have a finite amount of intellectual and emotional capital, just like we all have a finite amount of money in our bank accounts. It sounds like his conversations might be overdrawing your capital in this area. You seem to be in school, or at least studying, and probably busy in other areas. Kindly explain to your partner that sometimes his lengthy conversations can draw on your intellectual and emotional capital that you need to be putting towards other areas of your life. Also make sure to give him a way out, let he him know you value his ideas and would enjoy such conversions during a time when you have some extra energy.