What it means
How far? 多 before an adjective asks "how …?" — 多大 (how big), 多高 (how tall).
When to use it
Pair it with a destination: 到机场多远？ (How far to the airport?) Chinese answers are often in minutes, not distance.
What Chinese people actually say
Say this instead: 到……多远？(dào … duō yuǎn?)
多远 doesn't stand alone in natural speech — it needs a destination: 到机场多远？(How far to the airport?). Even then, Chinese speakers usually answer in time, not distance: 二十分钟吧 (about twenty minutes).
Examples
dào到nà那lǐ里duō多yuǎn远？
How far is it to there?
zǒu走lù路yào要duō多jiǔ久？
How long does it take to walk?
lí离zhè这lǐ里yuǎn远ma吗？
Is it far from here?
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.