There is not a limit to the length of time your batch can stay open, so take your time!
The only exception to this is when you are on the Proposed FBA Shipping page with a Private batch. This is because Amazon sets these shipping plans to expire after 48 hours, and the destinations for the items in the shipments are subject to change after that time.
To avoid issues, be sure to create all of your proposed shipments for a batch within that 48 hours period.
Or, if you get to the Proposed FBA Shipping page before you're ready to create all your shipments, you can always click the Decline and Go Back button to come back to this part later.
Any page before the Proposed FBA Shipping page, or after it, is able to remain open without a time limit.