Axes will be displayed in whatever units the plot is in. If you are in UTM-zone 31 in the North Sea, axes will display UTM-coordinates in meters. So - if you are plotting a large area, your axes increment should be set to e.g. 50000 or 100000.
The Axes and Frame menu
The image below shows and example of a plot with a frame and X and Y axes increment of 250 000 m. Notice that the plot also has grid +`es
Example of axes and frame plot