If you don't know javascript, it will take awhile. I know this because I didn't really understand javascript when I got started with it. By know javascript, I mean being able to understand and fully explain:
- Variable Scope
- Object.prototype
- .call, .apply, and .bind
- Closures/Lexical Scope
- Anonymous functions
- Function Hoisting
If you don't know all that, get and read Crockford's Javascript: the Good Parts
If you do know all that, then you need to understand how to use Backbone properly otherwise it won't make any sense. When using Backbone, you really shouldn't have any non-backbone javascript outside of a sparse initialization of your base Views...also learning how to use the built in event binding is essential. This is a pretty good guide, as (obviously) are all the sources here
So I would say it could take a few days to a week or longer to fully comprehend whats going on, and much longer to get to the point of being very skilled with it.