It depends on how you like to travel, but in 2 days you can see the main attractions in Seoul.

However, if you want to spend some time shopping, going to a spa or enjoying some of the restaurants and cafes in the city it would be better to add at least another day to the trip

Possible yes. Kind of a waste though. Wait until you have more time off then go. Korea is worth it.

I visited Korea for 4 days in December and I would definitely go again for the weekend. Seoul is such an awesome city and there's so much fun to be had! Unless you're planning on hiking the surrounding mountains its easy to do a quick tour of the main attractions in a day or two. Make sure you stay in Hongdae!

You will basically only have one day in Seoul. You will not be at your hotel in Seoul earlier than midnight, ans on Sunday you would need to leave in the afternoon. Seoul has pretty good transport, but still getting around the biggest city in the developed world does take time, so there would not be that much time to spend on playing tourist. 

As the others write, it's cutting it close. But if you desperately need to breathe clean air and don't mind spending the money, I think it can be done.

Seoul is certainly worth a visit. It's a strong contrast to China, possibly a bi boring compared to China. The people are nice, Korean BBQ is better in Korea. It's not a too expensive place to be, although I must admit the times I have been there, the cost of hotels could have been more friendly

You'd waste your money doing a trip like that. Just take leave and stay in Korea for 2 to 3 weeks on a real vacation and not a weekend getaway!

Hard to answer because everyone travels differently. I really prefer to relax and take my time while traveling. I'm there to see new things but also to RELAX, which means sleeping in late and being lazy. This requires more days to travel as a result.

You'd basically have one full day to really travel, and how much of that is going be you taking buses/taxies/getting lost, etc etc.  For the amount of money it costs I don't think it's worth it.

Everyone is different but if it was me who was planning the trip i would wait until some Chinese holidays come up so you have a few extra days. Or ask for some time off in advance.  Personally I would want at least 4 days to travel in Korea, but 5-7 days is my ideal travel time where I feel like I can see everything there is to see  but still actually have time to relax and not be rushing from tourist spot to toursit spot.

In my opinion Korea is a nice enough place that you should take a few days to really explore it, not just one full day and then a couple of short days with a lot of time spent on a plane