The bus tour itself takes approximately 1 hour and 11 minutes. 30 minutes from Salisbury to Stonehenge, 40 minutes from Stonehenge via Old Sarum back to Salisbury.
To find out how long it takes to visit Stonehenge itself, visit the English Heritage Website, there are tips on what to see if you have a limited amount of time.
We would advise that you leave about 5 hours to make the complete trip if you are visiting Stonehenge, Old Sarum and the Cathedral.