While I did search SP for this (frequency of new content) was a little surprised to come up a little thin on the topic.

Am trying to arrive at a broad rule of thumb, for how often new site pages should be added, over simply updating the site, though this is done as well.

So, for sites that are not themed in 'news', 'reporting' or the other intensely time-related, what might be a general recommended frequency for adding new content to keep things fresh? Understanding, that this is done for the visitors as priority, then, for the importance that the engines give to it. And too, that this must be maintained over the course of a given time period.