Does an adverb of frequency usually occur after the verb? How should I say "how often do you play sports?"
Actually a frequency adverb should be put before a verb. For example:
我(Wǒ)常(chánɡ)常(chánɡ)游(yóu)泳(yǒnɡ)。 （ I always swim.)
我(Wǒ)周(zhōu)末(mò)偶(ǒu)尔(ěr)去(qù)逛(ɡuànɡ)街(jiē)。( I go for shopping on weekends occasionally.）
And “ How often do you play sports ？” could be translated as: 你(Nǐ)多(duō)久(jiǔ)运(yùn)动(dònɡ)一(yí)次(cì)？( how often = 多(duō)久(jiǔ) ）
The second one is correct to express "How often do you play sports?"
BTW, I would like to give a comment to @Stan, that "你一次运动多久?" is better to express "How long does it last when you do sports for one time?". It's weird to say "你运动多久一次?".
The second one is better (as the first is a little grammatically off):
Although there's not really a hard and fast rule around whether it should be before or after the verb; from what I understand it should be based on the rest of the sentence structure. For example, these both work:
Perhaps it's best said that it goes before if it's being used as a descriptor for something (as in 多久 __ 一次) but not if it's used on its own / as the "length of time / how long" noun (as in 一次 多久)
[[nb. i might be terribly wrong. so take with a grain of salt like everything else on the internet :P]]