How long would it take to learn these languages?

I'm a native english speaker; I also speak German and some Spanish (I'm slowly learning more but it's not a priority for me). I'm unsure of which language I should fully concentrate on next. I've been thinking about: - Russian - Arabic - Mandarin - Greek - Polish - Hebrew - French -... show more I'm a native english speaker; I also speak German and some Spanish (I'm slowly learning more but it's not a priority for me). I'm unsure of which language I should fully concentrate on next.
I've been thinking about:
- Russian
- Arabic
- Mandarin
- Greek
- Polish
- Hebrew
- French
- Swahili

Considering, which one should I choose? How long would it take to learn that language?
5 answers 5