JPDM

Depends. At off-peak times the taxi will be faster by a good 30 minutes each way. And coming out of the Forbidden city the subway is not close.

CDKing

Much easier getting in. I didn't realize that i could not go back the way I came in wasting so much time.

JPDM

No, you enter from the south entrance and come out at the north one which quite a hike if you wish to get back to the south by walking around the FC walls.

YEGcg

seanpodge

There is security at the Forbidden City, but it's not full on airport style security. They're only interested in genuinely flammable/explosive/dangerous items (e.g. petrol, gas, knives, etc). On this, lighters must be handed over at the security check point, but there's a box at the other end where you can take a lighter, presumably from yesterday's batch (may or may not have any gas in it, try it first!).

Just looking at Baidu Maps, the train should take about 90 minutes. If you don't care about walking through Tiananmen Square (which also has similar security checks to the FC), you can get there a bit quicker and save your legs for the lengthy FC by transfering from Line 2 to Line 1 at Jianguomen station, then getting off at Tian'anmen East (Exit A). You'll then have a much shorter walk to the entrance.

At the other end of FC, it's a bit trickier. There are buses, but all bus stop information will be in Chinese. Also, you'll need a Yitong card for most/all bus routes. You can walk to the Dongsi subway station (turn right at the FC exit and walk straight ahead for about 25 minutes) and make your way to Dongzhimen and the airport from there. Otherwise a taxi to Dongzhimen is an idea too.

All taxis can go to the airport. Beijing traffic tends to be pretty bad though, so I'd still take the subway as much as possible. Much more reliable.

JPDM

All taxis can go to the airport. There is a problem with a taxi at the exit of the FC. There will be many touts that you should ignore and the taxis that are waiting at at exit are hoping for gullible clients (as most people coming out are tourists, Chinese or foreign). So they offer a fixed rate that is way over the metered rate. You generally have to walk for a bit to try to get a taxi.

seanpodge

Agree. Taxi drivers are generally honest in China, but certainly around any tourist areas, there are likely to be those who will offer/demand a (highly inflated) flat rate or are actually illegal "black" taxis. Walking away down the street for a bit should let you find an honest driver.

YEGcg

Hi all, here's a quick trip report and to add some datapoints, based on timestamps from my mobile phone photos. Again, thank you for all of your help!
Note: the times below are rounded, slightly. Also, after departing the FC we walked briskly so I have less timing points from photos, etc. The times that are italicized are approximate.

6:20 Land and clear customs, using visa line. Very quick and friendly immigration. Stop at ATM and coffee shop. Walk to Airport Express.
7:15 Arrive on Airport Express platform.
8:00 Transfer at Dongzhimen to Line 2.
8:15 Transfer at Jianguomen to Line 1.
8:30 Arrive at Security Checkpoint to Tiananmen Square, northeast corner, above Tiananmen East station. Walk to mausoleum.
9:15 Arrive at Tiananmen Gate entrance. Try to locate ticket booths (Not intuitive)
9:25 Buy tickets near Wumen (Meridian) Gate. Proceed through security. Walk through squares northwards.
10:00 Stop and warm up at coffee shop near Archery Pavilion, east of Baohe Hall.
10:15 Arrive at Treasure Museum, Clock and Watch Museum. (Extra fee each, bring cash)
11:20 Walk to Shenwu Gate and stop in souvenir shops.
11:50 Depart (walk) from Shenwu Gate east along Jingshan Front Street, then south along North Chizi Street. Stop at noodle shop near entrance to Duanku Hutong.
12:15 Noodles!
12:30 Walk to Tiananmen East station. Take subway east.
12:45 Transfer at Jianguomen to Line 2.
13:25 On Airport Express, nearly back at Airport.
13:40 Navigate T3, including train to international departures concourse.
14:15 Duty free and Air China Lounge
14:55 Depart lounge.
15:35 Flight.

Tips:

1. Stop at ATM in airport, before walking to Airport Express; I did not look for or notice ATMs any place after this. Airport Express appeared to be cash only, as are subway stations. There are nearly no commercial buildings near the north end of Tiananmen Square and no where to house an ATM. Also, "Cash is King" at the Forbidden City, including for the admission fees to the two smaller museums inside the FC. Souvenir shops accept credit cards.
2. The ticket booths are well inside what most people consider the FC / Palace Museum entrance at Tiananmen Square. Keep walking north until you see the security checkpoint, then look to the right at a small building with windows, near Meridian Gate.
3. Bring your passport to buy tickets on-site.
4. Smaller museums on-site, which charge additional admission fees, provide a discount for pairs. Try to avoid buying single person tickets.
5. Taking the Airport Express removed a lot of uncertainty for us, however it did reduce our time to look around. That said, I would recommend taking the Airport Express over a taxi, due to the traffic we saw along the highway.
6. Find a detailed map online to print and bring with you. There are areas to the west and east of the central axis where it is easy to become disoriented or lost.
7. If visiting in winter, dress very warmly. Almost all buildings are unheated.

Overall, 2.5 hours was long enough given the weather (sunny, -10C) for us to walk a brisk pace, see all of the major gates and halls, two of the feature museums located inside, stop inside for coffee, etc. We missed seeing most of the complex; the entire west side, southeast corner, and the area between Tiananmen Square and Meridian Gate. I would estimate we saw 20% of the complex and read 50% of the placards/signs we encountered. Walking back to Tiananmen East station was faster than I thought it would be; without stopping, it would have been a 30 minute purposeful walk.

I hope this helps others who are in PEK for a 9-ish (maybe even 8) hour layover. As with everything, YMMV!