Where to put the time phrase? 你运动多久一次？ or 你多久运动一次？
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) ）