Three or five days is enough for a good introduction to the main attractions of London, although this varies according to interest. For a shorter visit, visit the following: Buckingham Palace, Westminster, Big Ben, London Eye, Tower Bridge and Tower of London. Include extra spaces if time allows – museums, parks, shopping areas, neighbourhoods (e.g. Covent Garden). The Best Places to Visit in London (https://tourhopers.com/places/united-kingdom/london) can be spread out over the course of the entire day so you don't end up overloaded. It is easier to visit a fewer number of places well than a large number of places quickly. You can also plan your route before every day so that you can use your sightseeing time more effectively.