Cataract, the gradual hardening and clouding of the natural lens of the eye, typically takes many years to develop. It may be a decade or more from the time of diagnosis before the effect on vision gets bad enough to need surgery.
There are, however, some situations where a cataract can appear quickly and advance to cause a serious degree of visual impairment in just months: trauma and corticosteroid use are the most common of these types of rapidly developing cataract.