That would be „Od kiedy był profesorem?”.
Polish here is in present tense, because it asks about currently ongoing condition. Using past tense in Polish would suggest that either he is deceased or that he stopped being a professor at some point in time, which is rather unlikely. ;) While obviously the tenses aren't fully compatible between Polish and English, generally present tense should be translated with present tense.
I'm a native English speaker. It doesn't necessarily mean he is no longer a professor. However it can imply surprise/skepticism that HE is the professor or that he is a PROFESSOR, depending on stress (as can "Since when is he..."). Most often in English you would probably not use "since when" to ask a this question, but "How long has he been a professor?". It's a bit more natural than since when. But that might be a little too far from the literal Polish for duo.
When talking about time, "since" and "for" are used with Present Perfect, as in Duo's answer, not Present Simple (unlike in many languages).
It is possible to say "Since when was he a professor?", but it has a different meaning: it suggests that you don't believe he's a professor - "And since when were you such an expert on everything?"
For me the skeptical or challenging reading is the usual pragmatic implication with the present tense as well. As someone noted below, "How long has he been a professor?" would be a more neutral formation in English, although it asks for a duration rather than a starting point.
"Since when has he been a professor?" sounds thousands times better ;) you should use "have been +pp/have been -ing" and "since/for" for actions started in the past but still ongoing. I guess that "He is a professor since X years" is not only weird, but definitely incorrect. Any native speaker here?