Steve Simon has written a CIFS client for the Plan 9 Operating System (and has, no doubt, gained some painful experience along the way). He sent along a message with some interesting information regarding SMB ECHO:

"Though the SMB_ECHO packet looks like the perfect packet to send as a keepalive for sessions, it doesn't appear to reset the idle timer in either NT4 or WinXP."

Other SMBs (Steve used TRANS2_QUERY_FS_INFORMATION as an example) do reset the timer, thus keeping the SMB session alive. It makes some sense that the ECHO would not reset the SMB session idle timer, since it can be sent before an SMB Session is established (that is, before a SESSION SETUP ANDX or TREE CONNECT has occurred).

Steve also said:

"My cursory investigation shows Microsoft servers have an idle timeout of about 15 minutes."