how long is from Beijing capital airport to west railway station by subway?

Well, there is direct airport shuttle bus line 7 that can take you to West Railway Station directly. So you don't have to transfer to the subway.The operating hours are from 07:20 to 24:00. And the ticket fare is CNY 24 per person.
Generally speaking, the duration is about 1.5 hours.

If you wanna take airport express, after arriving at Dongzhimen Station, take subway line 2 to Xizhimen Station. Then take line 4 to National Library Station and transfer to line 9 to West Railway Station. It takes 35 mins with the ticket fare of CNY 5 per person.