It depends on how much time you're able to put into your studies, how often you practise using the language, and the degree to which you are immersed in it.
It is possible to acquire basic conversational fluency, i.e. the ability to understand and participate in ordinary conversations, in 6-12 months or even more quickly if you are immersed in the language and focus on speaking it. To acquire native-like fluency in a language is likely to take longer.
If your aim is to read a new language, you could learn to do so within a few months, if you are able to do plenty of regular study and practise. However acquiring the ability to read the new language as comfortably as your own will probably take quite a while longer. Learning to read Chinese or Japanese takes considerably longer than other languages as there are many more symbols to memorise.
To acquire native-like abilities in understanding, speaking, reading and writing a language, as well as an understanding of the culture of those who speak it, could take anything from five years to a lifetime.
No spoken language is significantly more difficult to learn than any other in absolute terms. After all kids can learn their mother tongues, whatever they may be, without too much trouble. However adults already speak one or more languages and generally find it easier to learn a closely-related language than a distantly-related or unrelated one. For example, the least difficult languages for English speakers to learn are Italian, Spanish, French, Dutch and German, in more or less that order.
Written languages are a different matter - some, particularly Chinese and Japanese, are difficult to learn even if you're a native speaker.
Each language presents you with a different set of challenges in terms of pronunciation, vocabulary, grammar, spelling and writing system. The closer these are to your native language, the less difficult a language is to learn.
The Defense Language Institute in Monterey, California, divides the languages they teach into four groups, from easiest to most difficult, as measured by the number of hours of instruction required to bring students (mainly native English speakers) to a certain level of proficiency. These are listed below: 1 = least difficult and 4 = most difficult.