According to multiple sources, Gran Turismo 5 takes up to 50 minutes to install.
"The game’s install takes about 40 minutes. You’re warned that it might be closer to 50, but Amar’s installation was done in ten minutes less than that, so it presumably depends a little on your model of PS3 and the hard drive you’ve got installed. The space taken is actually about 6.4GB, but that space gradually unpacks as you’re playing to be closer to the previously mentioned 10GB. In short, better make some room before the game’s release next week."