Which map software are you using? Apple Maps, Google Maps, Waze, Mapquest, etc...
Yes, mapping software will use data. How much would depend on how much you move around and more mapping data needs to be downloaded to your device.
I don't know about Apple Maps, Waze or Mapquest but Google Maps allows you to download maps for a certain area while you are on wifi to be used at a later time to save on cellular data usage. If you are worried about how much data your mapping software is using, you may want to look into this feature.
Your iPhone also has a data tracker on the device. You can check how much data your mapping software has used, then use the app for a few hours and check to see how much additional data it has used. That should give you an idea about how much data it is using per hour with your usage.
Using any navigation application while not connected to Wi-Fi will use your cellular data. The amount is dependent on the amount of information downloaded and uploaded while in use.