Certain landmarks require more time to reach than others, and some require additional time to complete sightseeing.  Due to this, there isn't a designated time for each attraction. Tour guides are knowledgeable and experienced and will modify their tours according to the tour groups behaviour and requests.  If you don't have much time at an attraction and you'd like more, reach out to the guide and they will see if they can cater to your request.  Remember, there's only so many hours in a day, so trust your guide - they'll help you make the most of your walking tour in London!