Might as well make a SWAG (scientific wild a$$ guess) because there's no such thing as a "common/standard ratio".  It's completely possible that someone has already made that decision and the entire population of what you're looking at are permanent (or at minimum long term) records.

As mentioned by others, thee are many issues, especially with oversized records, such as maps and drawings, to consider... and the age, form and format, of the source materials comes into play as well.  If they're hand drawn, possibly pen and ink on linen or vellum, of sepia 'prints' or blueprints, the text and line work may be EXTREMELY difficult up capture without using extreme measures and inter-copies.  Also, to remain useful, retaining scale on maps/drawings can be CRITICAL, which requires high level photographic processes.

The knee jerk concept of "convert or image everything" seldom makes sense in the end.  You may find a portion of these materials either have a short enough retention requirement, or are so seldom accessed and reviewed, that maintaining them in their current form/format, supported by a detailed index and finding aids is MUCH MORE cost effective.

Converting to film will result in a collection in media that is accessible for lengthy periods of time (if properly stored and protected), but will be expensive to generate... and you'll need some means of viewing and printing images. 

Converting to any digital form will result in a need to periodically convert and generate new, viable, formats... as well as storing them in multiple locations, to ensure persistent access over long periods of time.

Good luck convincing them to step back and consider their options... and if you're remaining in a position responsible for managing these information assets? Get their decision in writing. 

Larry

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