Monster Hunter: Wilds has an unusually short story mode compared to earlier titles in the series, as well as most JRPG. Chances are you will reach the end of the story in 12-15 hours, which may leave you wondering what to do next... and if it's worth it to keep playing. In our opinion, not only is it worth it: the best parts of the game come after.
To be honest, while the game is large, and there are new monsters and story beats, it's nothing game breaker: there are no new environments after the credits - well, except for one, and it's small. There are new monsters, and the difficulty increases exponentially (with Tempered Monsters), as well as the number of things you can craft (High Rank armour and Artian armour and weapons).
Most importantly, when the game asks you to "Investigate the regions and raise your HR" you will enter into the game loop that has characterised the series since the start: look for new weapons and armour to craft, see which materials you need and from which monsters do they come, and do quests hunting that monster. Multiple times if needed. Then, you will be better suited to fight tougher monsters. Or help a friend do all of that online.
The most likely doubt you will have during Monster Hunter: Wilds main story of after seeing the credits is how long does it take to beat it and how many chapters does it have. In total, there are six chapters in the game, and to see the main credits, you only have to finish the first three. You will finish Chapters 1-3 with HR (Hunter Rank) 8.
Don't worry, due to how the game caps your HR after story missions, you will start increasing your HR much faster after the first three chapters. The story is considered complete when you reach HR 40.
However, that is a little deceptive, as the final three chapters have very little story content, and even fewer cutscenes. If you want to see the game just for the story and characters, you will not miss anything major after the first three chapters. In other words, if you stop playing there, you will have got a good grasp of what the game has to offer.
If you want to stick longer, it is because you want to engage more with the mechanics of the game and aren't afraid of longer and tougher battles. Chapters 4-6 will ask you many times to simply "Explore the regions and increase you HR", which means that you have liberty to do any quests and hunts you want to increase your HR, which is now uncapped.
Chapter 4: You start with HR 8, and there are new story missions when you reach HR 15 and HR 20.
Chapter 5: You start with HR 22 and there are new story missions when you reach HR 32 and HR 34
Chapter 6: You start with HR 37 and you reach the final story mission at HR 40
After Chapter 6, you can keep increasing your HR with no more caps. There are new side quests, but don't expect new monsters. Well, except for the new monsters that will be added through DLC in the coming months and probably years.