There must be some trigger for your pupil dilation. Pupil is the camera aperture part in eyes, controlling how much light could get into eyes. In bright circumstance, pupil will contract, while in dime environment, it will dilate. No matter it dilate or contract, it is natural response to the light, to make us see clearer. Other common causes for pupil dilation include mydriatics, nervous system stimulation, drugs and poison. If your dilated eyes are caused by mydriatics which are used by doctor to exam your condition, the last period of dilation varies from case to case. Normally speaking, it can last from 24 hours to 2 weeks. With dilated pupil, you might found it a challenge to read or do some up close vision work. You also need to wear sunglasses to protect your eye when you entered a bright circumstances. If any trend to go worse, you should let your doctor know at once.