At what time do the afternoon tickets become available? Should I be in line waiting before this?

at 1pm. Not necessary to be in line waiting before that. My experience is that: although the museum is free to the public but some treasure are not in the basic display. And if you do not want to wait also, I suggest you pay about 20RMB per ticket to enter. When they told visitor the free ticket for the morning is sold out you can still the 20RMB ticket to enter the museum.