I'm in Shenzhen now, and need to spend 4 hours in Guangzhou this afternoon.

-The fast train itself only takes 30 minutes, but I'm not banking on getting one of those (they sell out), so I'm budgeting 1.25 hours plus 30 minutes wait time for this each way
-The required travel time on each end is 1 hour

That's 9.5 hours just to go to a single destination. If I were to try to add on a second destination in Guangzhou, I'd be able to stick to the 9.5 hour total, but would only be able to spend one hour in each place.

The same calculus applies in the reverse direction.

Decided to skip Shenzen and concentrate on seeing Guangzhou itself...

My flight arrives at 5:40am and departs at 7:10pm, i'm thinking of doing this bicycle tour that I found very interesting:

http://www.cyclecanton.com/

Unfortunately the tour starts only at 1:30pm and finishing at Huacheng Square around 5:30pm which will be cutting it close. My bags will be checked through and I would already have my onward boarding pass but I will have to go through immigration and security again.

IF I leave the tour a bit early around 5pm and catch a cab from Huacheng square (it's a Sunday evening) to the airport do you think I can make my 7:10pm flight??

I think it might be possible but quite tight.
7:10 flight would mean boarding start around 6:30. Taxi to the airport would be around 45min-60min so leaving the tour at 5PM gives you only a bit of leeway to find a taxi, account for traffic and immigration.
One alternative option is to take the subway from Zhujiang New Town, around 50 min to the airport but no worries about traffic. Plus the price at less than 10RMB is a bonus compared to taxi fare (120-150RMB)

Cycle canton got back to me and they think it should be fine leaving the tour at 5pm and getting back to the airport on a Sunday eve by 6pm. We'll see. I'm more comfortable taking the train since it operates on a predictable schedule rather than a taxi which can encounter traffic jams etc.

Latest development - China Southern changed my flight timings so now I have only a 5 1/2 hour layover in Guangzhou... arrival 5:40am and departs at 11:10am.

Should I just stay in the airport? I have been to Guangzhou airport and I really did not like it very much. Severely overpriced and nothing much to do.

The airport is not near any point of interest, and even if it was, given your time frame you won't have time to see much anyway. Not to mention, you'll probably be dealing with rush hour once you were downtown/heading back to the airport.

He could just ride the metro, though. Taking an early morning train into the city will still be relatively empty, and taking one back to the airport will be after rushhour, so neither direction would be crazy full.

A decent breakfast at the Pearl River might be a choice, especially if one is hungry after the flight.

I would probably decide on the spot on arrival if I feel fit for a few hours in the city or not.

Ok.. I guess I'll stay in.... I have a 15 hr layover on the way back so will explore the city then

well, I guess there is some confusion. My flight is China Southern 3067 on 11/16/2014. Orbitz sent me an email saying the schedule has changed and it's now departing CAN at 11:10am and arrives in KTM at 4:25pm. However, when I go into my Orbitz account it still has the original schedule of 7:10pm arriving KTM at 10:10pm. When I checked google and other flight schedules for the flight it says 10:10pm not 4:25pm per the "schedule changed" email Orbitz has sent me. I have already told my hosts in KTM to pick me up at 4:30pm based on the email from Orbitz

So, I went to Kayak and booked a flight on China Southern for 11/16 and well, it says arriving 10:10pm in KTM for flight 3067. Well, what is going on here...

I am not sure what is going on, calling Orbitz tomorrow. Why in the world would Orbitz send me such an email with such inaccurate information?