Usually the cache would get update only when needed. So, a new post is made, and the cache for that forum gets update with a new lastpostinfo. It isn't usually something that is run every 10 minutes or whatever. (Although, there are some that are like that if you set them to be like that.)
I did a search in the vb folder for lastpostinfo and found the stuff in the functions_forumlist.php file. You can read it there and see if you come to the same conclusion I did.
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