joystiq.com has news from sony saying that you can install EVERYTHING (eh.. by that i mean the whole installation) while you play. basicly it will install the stuff you're loading, so the first loading will take longer than the 2nd (unless ofcourse it loads up "new" stuff ect ect ect)
you can find the news yourself.
but basicly, theres no time wasted if you just boot up asap!
Totally depends on when I have GT5 in my hand. If I get it early on Tuesday it will be installed when im cooking and eating dinner. If I get it on the official releasedata it will be installing in the morning after I get it during a cup of coffee and some breakfast.
After unpacking and staring at my signature edition, I'm gonna be uninstalling my XBOX wireless wheel from the wheel stand pro, and fitting the G25 back there instead. It takes longer than you'd expect!
I don't know if I'll even put the game into my ps3 cos i don't have a wheel yet and it mite be a week after i have the game in my hands before i can get a wheel.