You are pretty safe to bet that the long-running musicals will be here when you come to London. Shows such as Les Miserables (the world’s longest-running show!) continue to play in the West End, alongside shows such as The Lion King and The Phantom of the Opera. Plays tend to have shorter runs in London although thrillers The Mousetrap (the world’s longest-running play) and The Woman in Black are mainstays of the West End.
If there are shows you are desperate to see it is always better to book in advance.
Some theatre companies run seasons of plays, allowing you to plan ahead for a whole season such as National Theatre, Bridge Theatre, Royal Court or The Old Vic. Other companies and venues may try to keep a large-scale musical running for many years. On average, most plays in the West End have a three to six month lifespan.