This value is used to timeout the given command. The units of this value are milliseconds. The time being measured is from when a command is sent until when sg is informed the request has been completed. A following read() can take as long as the user likes. Timeouts are best avoided, especially if SCSI bus resets will adversely effect other devices on that SCSI bus. When the timeout expires, the SCSI mid level attempts error recovery. Error recovery completes when the first action in the following list is successful. Note that a more extreme measure is being taken at each step.
the SCSI command that has timed out is aborted 
a SCSI device reset is attempted
a SCSI bus reset is attempted. Note this may have an adverse effect on other devices sharing that SCSI bus.
a SCSI host (bus adapter) reset is attempted. This is an attempt to re-initialize the adapter card associated with the SCSI device that has the timed out command.
The two error statuses containing the word "TIME(_)OUT" are typically _not_ related to a command timing out. DID_TIME_OUT in the 'host_status' usually means an (unexpected) device selection timeout. DRIVER_TIMEOUT in the 'driver_status' byte means the SCSI adapter is unable to control the devices on its SCSI bus (and has given up).
The type of timeout is unsigned int (and it represents milliseconds).
Whether aborting individual commands is supported or not is left to the adapter. Many adapters are unable to abort SCSI commands "in flight" because these details are handled in silicon by embedded processors in hardware. SCSI device or bus resets are required.