1. Depends on if the new language has ANY (or A LOT of) similarities to your native language or not. If you know English, you should be able to pick up Spanish, French, Afrikaans, Dutch, Italian, or Portuguese easier.
2. Depends on if the complexity of the new language. If you know English, German would be notch harder because of its grammar complexity compared to English but not too difficult either. Slovak/Vietnamese/Japanese/Thai/Chinese/Arabic/etc, however, are very hard.
3. Depends on the effort and other factors (teacher/tutor/time/money/tools/motivation). It can take years or as little as 3-6 months to strike a comfortable conversation with a native.
Refer to this link for your answer on the approximate hours to master the basic of each language.