When about seems rare (with the "approximately when" meaning). I did find one example in the British National Corpus from an oral history interview (formatting and punctuation added):
And er, so what, er, what, er, sort of era was, when about was this? still right at the beginning of the war when you were doing this, or ...?
That would have been, I would say, maybe August nineteen thirty eight.
I don't recall hearing when about often (eastern USA) but I am certain I've heard when abouts. I can't find evidence of this in any corpus I searched, but keep in mind that most of the spoken language samples available, for example, in COCA, are from news broadcasts. If your intent is to accurately represent colloquial speech, when abouts or whenabouts may be appropriate. It seems to be common enough in various dialects that people have questions about it, and there are plenty of search results (it seems common in forums). When abouts is also used by people on Twitter, usually in questions:
when abouts do you think you will be back?
When abouts would not be common where formal register is required. I probably wouldn't even use it in an email; I'd replace it with one of the more commonly written expressions pointed out by @Rachel or @drɱ65 δ. However, I wouldn't be surprised if, walking down the hall, my boss were to ask me, When abouts do you think you could do that?