When performing Ghusl Janabat under a shower I wash the body parts multiple times to ensure I haven't missed any area, and it takes me approximately 30 minutes to complete it. Is this reasonable or am I am being too careful?
Bismihi ta'ala
A ghusl itself should not take you more than a minute or two. Please keep in mind that when one excessively doubts and repeats their act multiple times, this will lead to waswasah, and simple things will become very difficult.
Please watch this tutorial on ghusl:
https://www.youtube.com/watch?v=JtDGChWoSeE
Also, on the topic of waswasah, watch this lesson:
https://www.youtube.com/watch?v=Pz2HGVepaC8&list=PLHj5BjXrLabga8Y6BJGjty6oNCzrwmXL-
With prayers for your success.