Time needed for complete urdf model?

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If you had access to the software (and had a working CAD model of your robot), you could use simmechanics_to_urdf to go directly to URDF.

As that might not be the case, you could try the Guardian stack. This is a four-wheeled robot with a hokuyo and accelerometer model already included. If your robot is skid-steered, the Guardian provides you with an example of how to model those physics in Gazebo.

For the bump sensors and the IR sensors, you might have to do some extra research.

Here is an example of a bump sensor.

Here is an implementation of an IR sensor. If that doesn't work, you could try this question.

Here is an example of the pr2's Prosilica camera that you could use.

Theoretically, I think you could build the model in an hour or two. However, that depends on a lot of things, chief among them, whether you know all of the measurements of your robot. Also, whether you build your robot from the simple shapes (cylinder, sphere, box) or want meshes.