There's no easy way to size /var as it's completely dependant on what apps you run.
/var is the default area for the www directory. Also mysql databases normally sit here. Squid defaults to using /var if you run it. As you pointed out, so does yum. It'll also depend on your log file rotation scheme (if you're doing any).
Unfortunately your question is akin to "how long is a piece of string"
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Advent 4211: Fedora 16