On this page of the guide to Mount & Blade 2 Bannerlord, you will find out how long it takes to complete the game.
Mount & Blade 2 Bannerlord is an action role-playing game with strategic elements. There actually is no end of the game - it is up to the player how much time they will need to conquer all the cities. The game can be "completed" (well, all cities can be conquered) within 40 hours. But it all depends on how quickly you want to build your armies and whether you want to go on conquests, or slowly explore the world, talk to its inhabitants, etc. However, Mount & Blade 2 Bannerlord is a game with an open world and no "real ending". In the game, you can spend over 100 hours and still not conquer all cities, or just play on even if you conquer them. You might as well finish the game a lot faster and start a new campaign by playing in a different way than before.