Is “by when” common? Is “by/since when” the same as “by/since which” here?

Those both sound very odd to my ears, the first a little less so than the second. For the first, I’d say “by which time,” and for the second I’d say “since that time.” “Since that time” would require a semicolon before it, though (or be the start of a new sentence).

No native English speaker in the US would say “by when” or “since when”. Both sound awkward and unnatural to my ears.

Oh yeah this feels fine to me as a native speaker in New England (Northeast USA)

As someone who likes talking fancy sometimes, there are some situations in which I will flip my sentences (as seen here). "By/since which" is not a structure that even I use. I would much rather separate that into 2 sentences. The baby is due in May. By then, the house should be finished.

This is not correct. "Since when..." can be used in a question, although using them would immediately make me think the question was not entirely serious. E.g. "Since when have pigs flown?" It's a rather sarcastic means of pointing out that one doesn't believe something. In both examples of your question, I would "then" in place of "when".

These sound natural to me in Australian English.

A: "I have a job that I need you to finish for me." B: "By when?" (Used to clarify the timeframe) A: "I won a marathon race yesterday. " B: "Since when did you take up running?" (Used to express surprise)

It sounds pretty unnatural to me for American English. I would phrase both of those as two sentences. The baby is due in May; by then, the new house should be finished.

I would not say "by which" in this way. I might say "by which _time_", "since which _time_". Or date, day etc.

American English speaker here. I would understand these in context, but I have personally never heard or used 'by when' and 'since when' in this particular way.

"Since when" sounds fine to me. "By which" would have to have "time" after it to be equivalent.

Toronto here. I say this rather frequently.

No. “The baby is due in May, by which time the new house should be finished. Or “The baby is due in May. By then the new house should be finished.” “That was written in 1946. Since then the education system has undergone great changes.”

I have never heard either of those uses in that image.

The “by when” example is fine but the “since when” isn’t how a native speaker would format it. A native speaker would more likely say: “The education system has undergone great changes since it was written in 1946.”

Neither of those feels right to me. I've never heard it this way. If I was going to use a similar wording, I would replace *when* with *then.*

It makes sense to me if I think about it for a while. Definitely not standard in conversation or casual.

Yeah never heard of these. It sounds weird but I could see some old or formal writing using it. Like you said normally the top sentence would use "by which time" or "The baby is due in May, the new house should be finished **by then**."