Hello Alex,
It all depends on the accuracy you are trying to achieve,as you may already know that with increasing distance the accuracy of the object temperature lowers (unless we use a lens based approach). Distances <6 inches should be able to provide you good accuracies(<2 deg depending on the setup), as long as the object occupies its entire field of view.
Since airflow, temperature range, object's emissivity and size of the measure area play a major role in accuracy of the IR temp sensor, I can provide you specific answers if you can explain more about your intended application.
Best Regards,
Abhi