The question is: How interested in this gig are you? 1 year is plenty of time if your willing to really focus and get it down. You already play guitar and if that's something you're more interested in you may not be willing to focus on playing bass seriously.
To answer your question though, I'd say a year is more then enough time to learn what you need.
To answer your question though, I'd say a year is more then enough time to learn what you need.