How often were fire arrows used in medieval combats?
Fire arrows were used far less in Medieval combat than is shown in Hollywood films or other representations. In general, fire arrows were not used in pitched battles purely due to the fact that they were not worth the trouble.
Having enough fuel for these arrows would take a considerable amount of resources and usually the flame would go out when the arrow was in the air due to wind resistance.
Fire arrows were sometimes used in sieges, but again these were quite rare and in general defenders preferred to use boiling tar or oil instead of fire arrows as a form of incendiary defense. In all, fire arrows were likely to be rare in Medieval battles purely because they were just not worth the effort.