In the 6000 Series Programmer's Guide, page 53 and 59, it states that the RecvAT6400Block function returns a -2 if it times out. This is incorrect. This function does not have a timeout, and will return a 0 (indicating 0 bytes) if the block is not ready to be read. The functions which do have a timeout associated with them are:
SendAT6400Block
SendAT6400File
request_status
GetFastStatus
GetExFastStatus