"This flight will only last two hours."
Translation:Chuyến bay này sẽ chỉ kéo dài hai tiếng.
It is an alternative for giờ, usually used in informal context. The word is only used to refer to the duration of time, not the time itself.
It derives from tiếng đồng hồ (rings from a clock) as older clocks, especially in town squares would ring on the hour. It's very commonly used colloquially by Southern Vietnamese.
Yes, another new vocabulary item in a too-large list of 32 in this module that could have been saved for later. The problem with DL Vietnamese modules is that they contain way too much to digest and retain properly, not too little.
Is this sentence also ok? (it was rejected) 'Chuyến bay này sẽ kéo dài chỉ hai tiếng.'