An hour and a half is not too long at all. Sometimes turtles will bask for four or five hours at a time. Sometimes Spot will bask on and off all day into the night and still be up on his basking platform when the timer turns the light off.
When a turtle is sick he's usually lethargic and then he may bask a lot, but being sick and lethargic is not the same as a healthy turtle that is just enjoying his basking. When a turtle is sick and lethargic he's slow most of the time, not interested in his food. If he's the kind the jumps off the platform into his water when a person comes too close, if he's sick he may not jump off but just sit there. You should be able to tell the difference whether your turtle is sick and lethargic or if he is just enjoying his basking.
