I'm a native English speaker, who now also speaks, reads and writes Mandarin. The number one most important thing in my opinion is regular, consistent practice

I'm a native English speaker, who now also speaks, reads and writes Mandarin. The number one most important thing in my opinion is regular, consistent practice over a sustained period of time. So long as you are using decent materials (there are plenty for whatever language you are learning) and as long as you have incorporated some sort of feedback mechanism to spot mistakes (recording yourself speaking, speaking with native speakers), you'll slowly but surely make progress. Regarding time, I'd say it took me about 5 years to get comfortable with the language (including reading/writing) such that I could conduct myself in Mandarin in a given situation without worrying that my language skills (or lack of them) would trip me up.