How much time should I allow for climbing all of the steps of the Duomo, taking a look at the wonderful view of Florence, and going back down? I really want to do the climb if it not too taxing for a 72 year old.
You might find a bit of a line to go up, we went by the other day and there was a line from the side entrance all the way to the front of the cathedral. So that might take a while... unless you get the Firenze Card, there is a priority entrance for that.
I'd allow at least an hour for the climb up, time to enjoy the view, take pictures and then climb down. There is no one to hurry you so you can take your time. Stop at the base of the drum of the cupola inside and enjoy the close up view of the frescoes inside the cupola itself of the Last Judgment, for instance. Unless you have specific problems I don't think you should have any problems!