I agree with Lurcherlover. If you are not prepared to be out for hours in all weathers, dog ownership is not for you.
Other than that it very much depends on the dog and the type of walk.
I have a Fox Terrier x and if we are just walking then he needs at least an hour. If I take him for a run he is happy with 30 mins. If he is runnning alongside my bike then he's usually ready to go home after about 15 mins, but he will expect another 20-30min walk later in the evening.
He also does the school pick up run with us.
Walking is not the only time consuming thing you need to consider. There is training and play too. Terriers are very intelligent dogs and need stimulation to prevent them from becoming bored and fustrated.