In general, the meaning would be different, or at least ambiguous:
...jusqu'à six heures: until six o'clock.
...jusqu'à six heures durant: for up to six hours.
As commented, it wouldn't make sense for a thermos documentation to specify a precise hour of the day until when tea is kept warm inside it so the meaning of the sentence without durant would be understood exactly like the longer one.
A different context might make distinct meanings more likely :
Il a dormi jusqu'à dix heures. He slept until 10 o'clock.
Il à dormi jusqu'à dix heures durant. He slept for up to 10 hours (in a row).
Note that the English during evolved from the French durant, keeping the same meaning.
Durant is formal in French so we usually prefer pendant but the latter can't be postfixed :
Durant six heures. ✓
Pendant six heures. ✓
Six heures durant. ✓
Six heures pendant. ✗