That still doesn't sound natural to me. You could answer 'since when' with 'two days ago', but declaring it as a statement I'd say 'for the last two days' or 's

That still doesn't sound natural to me. You could answer 'since when' with 'two days ago', but declaring it as a statement I'd say 'for the last two days' or 'since the day before yesterday'. You could get away with it in speech since it sounds like 'since.. [thinking] two days ago' and it's acceptable to change construction like that in casual speech, but written it doesn't seem right to me.