I only use "c." with dates, usually:

circa prep (abbreviation c.) used especially with dates: about; approximately • circa 1250. http://www.chambersharrap.co.uk/chambers/feat...

In a scientific text, I'd probably use "approx." or "approximately", and in non-scientific texts I might use "about" etc. - http://thesaurus.reference.com/search?q=appro...

I suspect your policy is a good one.

I needed this as a morale boost.

(But I still think approx. is ugly.)

If an abbreviation offends your eyes, then you have a better sense of how English speakers feel reading German, which tends to be peppered with (in our view unnecessary, and unnecessarily confusing) abbreviations.

Not only 'approx.' but almost all abbreviations for actual words are generally avoided in formal writing in English. The few exceptions are extremely common abbreviations like 'etc.,' but even those should be used only sparingly. (A rule I break a lot in casual writing, like here in the forum, but still.)

I wouldn't use 'approx.' even in scientific writing except in a table or something where there was really no space to write a long word. Why abbreviate when there's no need to?

I agree that 'circa' is used only with (historical) dates. It is normally abbreviated, since it's thought of as similar to a unit of measurement, rather than an actual word. I myself would probably abbreviate it 'ca.' rather than 'c.'

I agree with hm and CM2DD.

I do a lot of technical translation, and generally I write "approximately" where the German text says "ca."

Agree with hm and Maria E. - I write "approximately" in full whenever possible and c. or ca (circa) only with dates.

Most medical/scientific publications count words, not characters - both approx. and approximately count as one, so there is no point in using the abbreviation ...