subway travel times in Beijing

You mean you want to take the high speed train to Xian?
In my opinion, you can arrange your terracotta trip in two days, so that the time is not so tight.
On the first day, you can take a high speed train to Xian and then wander around the Xian city proper and see the City Wall.
On the second day, you can head for the terracotta army in the early morning, so that you can be back to the city railway station in the late afternoon. Then you can taken an overnight train to go back.

If you only use one day, you will reach Xian in the noon time and time will be tight to see the army and back to the city proper.

It is suggested to do the trip in weekdays, so that it will not be crowded.
The subway takes about half an hour to the west railway station.

From the terracotta to the airport, the fast and best method is direct taxi if the time is tight. If you are not in a hurry, you can take the local Tourism bus line 5 to back to the local railway station, where you can find the airport shuttle bus line 2 to the airport.

The shuttle bus fare is around 26RMB. The direct taxi costs around 200RMB per trip. The toursim bus line 5 costs 7RMB per person. For the train's fare, you are suggested to look in the Train Section on this website.