I think these 6 months were for the entire tanksystem in RO2.
For creating a vehicle you need to keep in mind that we have many steps, which in most cases can't be done by a single person.
1. Research
- collecting photos, blue prints of the exterior and interior
2. Creating the model
- exterior and interior need to be created
3. UV mapping the model
- for the upcoming texture process we need a UV map
4. Texturing
- for texturing a vehicle you need many different maps (diffuse, specular, normal, more)
5. Rigging the model
- the model needs bones for the further process
6. Animating
- the vehicle needs animations, additional animations for the crew are needed as well
7. Code
- the vehicle can be added to the game now and gets scripted
8. Testing
9. Improving
10. Testing
.... and so on
You see, for getting a functional vehicle there is much to do.
Also keep in mind that the time changes for every vehicle and it's also different for each modeler and so on.
Best regards,
Blitzkrieg-Bob