I have installed Ubuntu Precise since it is a Long Term Support Release than Quantal which is the most recent release of Ubuntu. Another thing that you should aware before to install ROS is that you have to choose among the three releases which are Electric, Fuerte, and Groovy. Because of the mass migration of code to GitHub, I decided to install ROS Groovy Galapagos. GitHub has involved the ROS community more and given it more ownership of the codebase [1].
A detailed description of the installation is given in the following link http://www.ros.org/wiki/groovy/Installation/Ubuntu The desktop-full installations takes around 1 hours.
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