When will a question be closed in Stack Overflow, i.e. transform from "on hold" to "closed"?
Questions remain [on-hold] for 7 days.
This is something that can be changed per-site, though they are all defaulting to 7 days at the moment.
Update: The "on hold" period was briefly set to 7 days, but it has been changed back to 5 days, which was the original intent. This time period is set to match the window during which an edit by the OP sends an "on hold" question to the reopen review queue.