Quand on fait de longues gardes fatigantes

Bonjour James,

Here "de" refers to the partitive article des (some) which becomes de (or d' in front of a vowel or mute h) in front of an adjective (longues)

Take a look here: "Des" becomes " de/d' " in front of adjectives preceding nouns (French Partitive Articles)

I hope this is helpful.

Bonne journée !