When taking a bath the longest that you should stay in the bathtub is 30 minutes.
If you stay in the bathtub for longer than 30 minutes you can cause your skin to dry out and your skin will become wrinkled.
When your fingers begin wrinkling while in the bath tub it's a good indicator that you're staying in the bathtub too long and should get out of the bathtub before your skin starts drying out.
The fingers and skin will return to normal after a few hours or so of getting out of the bathtub.
Sometimes I love to take a bath and relax in warm water with a bubble bath and I would love to just lay in the bathtub longer and relax but 30 minutes is long enough for me and by that time the water is starting to cool down anyway.