it looks like were going with 10AM PST or 1PM EST and the GM download was like 780mb so I'm sure it'll be right around there so time of download will depend on your internet connection
But i'm pretty sure it's all safe as with most updates like this iTunes will firstly download the update before installing. So if it crashes on download, all is fine. It's if it crashes when installing were there could be problems but i would say that would be Client side issue's if anything.
I'll be checking iTunes most of the day if i'm being honest >_>. Hehe. Good luck everyone