We often get asked that question. Unfortunately there is no simple, generally applicable answer.
According to data in this paper, taken from 3611 veterans in Australia, 50% survived for 30 months after starting on ADT. But many men survive much longer. However, keep in mind that the men in this study were older (mean age 83) and had more comorbidities than is average for men of their age.
Gadzhanova S, Roughead E. Survival after initiation of androgen deprivation therapy for prostate cancer of Australian men. Cancer Epidemiol 2015; 39:854-9. http://www.cancerepidemiology.net/article/S1877-7821(15)00228-3/abstract