We don't know yet. Under the original plan/Update 81 sent March 26th they indicated addresses would lock at April 5th and at some shortly stage after that we'd be be able to pay up until April 12th 12am ET when the pledge manger would lock, so about a week or so. But things have changed so no idea if there will be a similar or longer period now.
To clarify a bit further, in Update 85 (
where they explained what went wrong), they removed the April 12th deadline completely:
"Finally, the April 12th deadline is removed. We will take as long as it needs to fix all issues."There won't be a new payment deadline until they've fixed all the shipping issues, which will presumably be communicated via a Kickstarter update.