When you go to have an eye exam, you will take several tests to determine how your prescription is and whether you have eye health problems. Usually the eye exam will last around 20 minutes. Some are even shorter about 10 or 15 minutes. But some people will spend a longer time on the eye exam, maybe about an hour depending on whether you need to have a comprehensive eye exam.
If you take eye exam for eyeglasses, it wouldn't take you much time. I remember that the last i taking eye exam cost less than half an hours. But i also heard that some people it depends. If you want to take an comprehension eye exam, it may takes more time. Anyway, you can try out tomorrow. And if you want to buy eyeglasses, you can take prescription to order it from some reputation online shop.