To find out how much filament is needed to make an object:
1. Run the object's stl file through slicing program so that the Gcode is generated. You will have to input set up the slicer to suit your printer's specs, which is a task you would have had to do anyway.
2. Run the Gcode file through interface software such as Pronterface, or similar. When this software has worked through the Gcode, it will tell you how much filament will be required to print the object.

Old Man Emu