There is not a limit to the length of time your batch can stay open, so take your time!
The only exception to that is once you have gone forward in the Private workflow to the page that shows your Shipping Plan (It's called the FBA Shipping page), you will want to decide which shipments you intend to send to Amazon and get them created to push on to the next part of the workflow. This is because Amazon sets these shipping plans to expire in as little as 48 hours.
Any page before this one, or after it, is able to remain open without a time limit. :)