merci but if you would think.... how many of the minutes... as understanding the grammar... not that you would say it in English... then ...." des'.... would be the pronoun to minutes relating to plural minutes. And not be the article for minutes but express the genitive. however it is easy enough to learn combine de ......
The following adverbs of quantity usually follow the structure:
adverb of quantity + de + noun without an article
assez, autant, beaucoup, combine, moins, pas mal, peu, un peu , un petit peu, plus, tant, trop
ils ont un peu d'eau.- They have a little bit of water
Il y a beaucoup de problèmes. - There are a lot of problems.
Encore plus de problèmes. - Still more problems.
Il y a tant de problèmes. - There are so many problems.
The following adverbs, are always followed by the definite article.
bien - quite a few
encore - more
la majorité - the majority of
la plus grande partie - the majority
la minorité - the minority of
la plupart - most
une quantité - a lot of
La plupart des gens aiment notre président. - Most people like our president.
La majorité des maisons ont des jardins. - The majority of houses have gardens.
There are several such expressions which always include "de". Combien de, beaucoup de, moins de, plus de, trop de, etc. Here is a link to a full explanation: http://french.about.com/library/weekly/aa060300q.htm
The real question is not "why" for every language has its own rules when it comes to prepositions.
You have to learn that the preposition "de" is used in many grammatical constructions and very often when it comes to quantities:
- combien de... is the way to ask about "how much" or "how many"