Gran Turismo 5 optional install range anywhere between 256 MB and a few minutes for the basic installation to around 10 GB and 50 minutes for a full install.
The benefits for the 10 GB install is mostly to speed up waiting for an online race and quicker loading screens. Both benefits can be negated if other racers in a track don't install before racing online since the installation is not mandatory.
More space is needed due to the game installing/unzipping itself while you play since the initial install only uses up 6GB and then does the rest while playing.
http://kotaku.com/5694720/gt5-instal...-hour-to-spare