How long would you recommend at 798, if I want to go through at a moderate pace? Is there an admission fee?

Also, when does it get dark in Beijing? I will be there 1st 2 weeks of December.

Finally, I see people asking about bus stop names. I know in some cities (not mine), the bus has an electronic banner that displays the stop as you approach it, so it's easy to know when you need to get off. Do the buses in Beijing have these?

Thanks!