This is a tricky question. Mainly because if you have a set of 24 pastel pencils, you are only going to be using half a dozen of those pencils regularly. You will naturally use them all but one or two of them here and there. The white and the black are colours most frequently used. For me, one white pastel pencil would last me 5 or 6 pictures and that is me using white very often in A4 sized pictures with skies. A light blue on the other hand could last me around 20 pictures. If it was a red it would last me 30 pictures.
This is one of the great things about pastel pencils, there is no real waste as with other mediums. The pastel is blended in with the ingres pastel paper and with the faber castell pitt pastel pencil range, the leads don’t crack very often when sharpening due to the manufacturing quality.