I was hoping mokujin might decide to come up with something. Anyway, it's actually a pretty simple plugin, you just need to create a new plugin using hook postbit_display_complete and code something like:
Code:
if (!$post['posts'])
$post['era'] = "No Posts Yet";
else
{
$days = $post['joindateline'] / (3600 * 24); // 3600 * 24 = seconds in a day
$era = $days / $post['posts'];
$post['era'] = vb_number_format($era, 2);
}
Then use {vb:raw post.era} in your postbit or postbit_legacy template.
This just uses 1/posts per day so that lower is "better", but I'm kind of guessing that's going to give values for some users that don't look like reasonable ERA values, so you'll probably want some tweaking.