"Since when do you use email?"
Translation:Ekde kiam vi uzas retpoŝton?
According to the rules about participles given to us in the “Education” tips and notes section, I think that to translate the present perfect English form to Esperanto you should use the active participle in the past tense. “Since when have you used email?” would become Ekde kiam vi estas uzinta retpoŝton? I know that these sentences and the original ones convey similar meanings and therefore can be interchangeable most of the time, depending on the context. Regardless, IMO the key here is that they do not mean strictly the same, because the present perfect, however close it places the action to the present time, belongs to the past tense and cannot be translated as a present into Esperanto.
> Ekde kiam vi estas uzinta retpoŝton?
Esperanto compound tenses are "compositional," meaning you can split them between the main verb and the participle and understand each separately.
> Ekde kiam vi estas | How long have you been
> uzinta retpoŝton? | someone who once used email?
In a present-tense context, yes. And questions are pretty much always in present-tense context.
(The one exception I can imagine is a rhetorical question in narrative prose. "What would happen to our heroes next time?" Or, as Esperanto might phrase it: "What happened to our heroes according to the next telling?")
But a relative clause could be in narrative context like this:
> La doloro komenciĝis proksimume tiam, ekde kiam ŝi uzis retpoŝton.
> The pain started about since when she started to use email.
"Since when did/do you use email" always sounds a bit humourous or sarcastic in English - it means that the person in question rarely uses email and the questioner is surprised to see them doing so. Is that what's intended here? Or does it mean "When did you start using email?" or "For how long have you used email?", which are more neutral and factual in tone?