Can be done in a day!
We arrived in Pisa at 6pm and left the following day at 2pm, this was ample time to do everything! We had a nice meal in the evening, saw the tower, went up then tower and went to the cathedral the following morning, toured the local markets and had a nice lunch! The city is a little grotty with the exception of the main tourist area.
Insider Tip:Stay at the Window to the tower B&B! Its easy to find and walkable from the station, get to the river, cross it, head to the tower and then you are basically there!