To calculate the rate of a flow, the bytes/second which a flow is using, the duration of the flow is required. To calculate this opFlow uses the starting time of the flow and subtracts it from the flow finish. This provides a number of seconds the flow was running for. Because single packet flows have a duration of 0, as that is all the network device see's opFlow has implemented a default flow duration as a configuration option.
The default value for this is 0.1 or 100 milliseconds, this can be changed with the configuration option "opflow_default_duration"