Red Eared Slider? If this is a new change in behavior it might indicate the beginnings of a URI. Turtles often try to raise their body temperatures and bask for extended and unusual amounts of time if they have upper respiratory infections.
Do you have a water heater? If the ambient temps have changed recently then the water will be colder -- it may also cause the turtle to bask more.
Have you noticed any other changes? Amount the turtle eats, etc?
I suggest you seek a herp vet and see what he/she thinks.