I wasn't able to find this (when I implemented the delay), so I use a hardcoded value. If anyone has info about how to obtain the system-wide preference for this, I'd be happy to use it.
Documented in Xcode "Expert Preferences" release note, strangely enough:
NSDragAndDropTextDelay
The delay (in milliseconds) during which Xcode performs a drag-and- drop operation, rather than a selection. Setting this default to a non-positive value will disable drag-and-drop editing.
So, 10 second delay system-wide: defaults write NSGlobalDomain NSDragAndDropTextDelay 10000