(source:sciencing.com/minimum-oxygen… & anatomy textbooks)
The average lung capacity of an adult male is 6L btw
If Fleur and Cedric had 10L of air containing 21% oxygen, they would inhale and exhale roughly twice before all the air in that bubble had been in their lungs, after which point the air would be roughly 16% O2.
Now to be fair Fleur gets attacked and turns around after only about 15 minutes (if I remember right), but even that time is too long for her O2 supply. Humans at rest breathe 12-20 times per minute btw.