Replying to Is the following considered 1 IM conversation or 2 IM conversations?

Is the following considered 1 IM conversation or 2 IM conversations?
Person A opens a conversation by IMing Person B at 9:15:00PM. The two have not spoken previously that day.
The conversation continues until 9:31:33 where Person B puts up an away message. At the same time,at 9:32:00, Person A IM's Person B.
Person B then goes to eat dinner or something.
Person A goes to watch the redsox. Person A closes the IM window.
2 hours later at 11:14:46 Person B responds to Person A's last IM. 
 
Would you consider this to be 1 "IM" (aim conversation) or 2 "IM's" (aim conversations)?