French phrase
Depuis quand constatez-vous ~ ?
Meaning
Since when have you noticed ___?
When to use it
It asks when a problem or change was first noticed. The tone is polite and somewhat formal, often used to confirm when an issue began.
Example dialogue
Depuis quand constatez-vous une baisse des ventes ?
Since when have you noticed a decline in sales?
Depuis le mois dernier.
Since last month.