Refer to the subject.
Also I want to ask how to WRITE. It's a question beyond language proficiency, it's about creativity. But I want to ask if BIG words help 'furnish' one's written work? or instead, it only makes them look redundant?
It's hard to answer your question without stating the obvious ("It depends on the person and the language"). I think I could master any European language in 2-3 years by learning every day or almost every day. By "master" I mean "reach a near-native level of proficiency".
As for your second question, it's ordinarily better to use simple words than complicated words, but sometimes you want to impress other people with your English.
There are a lot of issues here to take into account, for example...
- The quality of your learning method
- What you mean by having learnt a language
- The similarity of the target language to your native language and/or other languages you have learnt
Based on learning for an average half an hour a day, I would estimate it would take about two months to reach "survival" level, about six to nine months to reach "conversational" level, and probably around two years with some time spent in a native culture to reach "full" fluency. This is if you're using an effective learning method, with inefficient methods these times would probably double.
By "survival", "conversational" and "full" fluency I'm referring roughly to levels 1, 2 and 4 of the ILR skill levels (see
http://www.sil.org/lingualinks/LANGUAGELEARNING/MangngYrLnggLrnngPrgrm/TheILRFSIProficiencyScale.htm )
I don't see "native" fluency as a realistic goal for the majority of learners.