What do the French words 'depuis quand' mean in English?

The previous answer given was simply since when, which is correct but not complete.

For an English speaker cannot simply insert depuis quandin the same places as an English sentence when trying to express an idea since in French. However, I would like to first draw your attention to the use of depuis, when not combined with quand.

We use depuis with the present tense (présent) and an expression of time to indicate that an action began in the past and is still occurring presently.

Je cuis le rôti depuis hier soir.

(I have been cooking the roast since last night.)

Elle fait d'exercise depuis trois mois.

(She has been exercising for three months)

Depuis with the present tense has two senses, or meanings:

-It means since when you add a day/date

-It means for when you add a length of time

We use (depuis quand) in a few ways:

We use it in correspondence to the above examples; depuis quand is used in interrogative statements (questions) to determine the precise date from which something has occurred.

The expression combien de temps compliments depuis quand, as it requests a smaller amount of time, such as hours or minutes. It usually indicates a shorter temporal period, but the person asking the question should use a form which reflects their perception of how long something would normally take.

Example:

Depuis combien de temps cuis-tu le rôti? (For how long have you been cooking the roast?) Note: Roasts aren't usually cooked longer than a few hours so the question would reflect this temporally, using depuis combien de temps.

Depuis quand fait-elle ses exercises?

Depuis combien de temps fait-elle ses exercises?

This example demonstrates the difference between the two, as it could have two senses. The first question would be appropriate for determining when she began her regime, or started her new habit of exercising. The second question would make sense in asking when she started exercising today, this afternoon, at what time she started exercising etc.

We also use it with the imperfect tense (l'imparfait) to indicate an action that began in the past and continued until it was interupted by another event.

For further investigation see: Depuis quand avec l'imparfait