A rough indication of the delay in monosynaptically connected cortical neurons is 6 - 14 ms.
As @Jonathan correctly states, there are many variables involved in neuronal delays. However, because you ask for an estimate of the order of magnitude of the neuronal delay between two coupled neurons in the neocortex, I will suffice by providing the latencies reported form one study that seems to meet your demands very well. In this study the latency data on horizontally connected neurons in monkey prefrontal cortex in layer III were reported González-Burgos et al (2000).
The authors inserted stimulating electrodes to evoke action potentials and used distant recording electrodes to measure the responses in cells located distally from the stimulation site (Fig. 1).
Recording setup used in González-Burgos et al (2000).
They report that delays between monosynaptic connections range between 6 - 14 ms. Fig. 1 shows that these latency estimates depend on the position of the stimulating and recording electrodes, i.e., which part of the presynaptic cell is stimulated and which part of the postsynaptic cell is recorded. Most notably in the recording setup used, longer axon collaterals will have resulted in longer latencies. The authors conclude that the conduction velocity of the horizontal axon collaterals was 0.14 ± 0.04 m/s and the synaptic delay was estimated at 2.29 ± 0.49 ms.
- González-Burgos et al., Cereb Cortex (2000); 10(1): 82-92