This kind of evaluation can indeed tease out various psychological or psychiatric issues - distinguish between this one and that one. The good news about that is that it can guide treatment more accurately.

How long the testing is depends on how far he wants to go with it. If it's really thorough it can take several different appointments. It can include some type of intelligence testing, personality testing, projective testing (like Rorschach), and interviews.

I'd ask him for more details.