You will need to edit a file to do so. Just remember that Edits(Hacks) are not supported by the vendor.
In your UBB_JS folder in a file named boxover.js. Edit that file and search for curNode.delay=
It is currently set to 0. This is a value in Milliseconds just increase it to what you want for a delay. Also this script is borrowed from
http://boxover.swazz.org/ which you can see at the top of the file. So if you need more info just visit that site.
I don't know if the delay is cross browser compliant but it works on IE7