Close entry editing after set time (for certain a user group)
Seems like you'd just need to write a plugin that listened to the onBeforeSaveEntry event.

That plugin would check if the entry author is in the user group you're interested in, and if it's a new entry just let it save.

If it's an existing entry, check the dateCreated date against your window and see if it's still valid.  If it's outside of your window you can set performAction to false so the entry won't save and set a flash message indicating they're outside the window.