The default wall thickness is 25 cm or 9.8 inch. If you wish to change it, you should do it before you start drawing the walls. Click on the Settings and enter the new value. You can also switch between centimetres or inches as a unit of measurement.
In case you need to draw walls with different thickness - for example, external and the internal walls, you can change the default thickness before you draw every type of wall. For example, first do the external walls and then change the default thickness for the internal walls.
Another possibility is to change the thickness of each wall individually. Tab on the wall you wish to edit and select the i-button. Enter the new value in the ‘Width' field: