it depends on the language and the amount of effort the person puts in, some languages take a lot longer to learn because of the amount of grammar rules etc but other languages may not take as much time because it may be quite similar to your own language. You could learn a language fairly fluently within a year if you practiced everyday and continuously learned new vocabulary. As long as you work for it.

Exactly like Lucy said. Some languages take more time to learn depending on their grammar rules / variation. But it's practice and hard work what makes you learn fast, and motivation.