I shoot for a minimum of 90 minutes before a race. I've been to races where it took more than 30 min. to get through registration - and that was for pre-registered riders. I like to have plenty of time to register, chat with fellow racers - at this point, I know half the people at about every race, hit the bathroom, go over the bike one more time, get kitted up, warm up, hit the bathroom again, chat some more, warm up some more (I find I do best when I've had at least 30-45 min of easy spinning to get myself warmed up)....
But that is just me.
And Rkwaki's suggestion of counting backward is what I do: Race starts at 2. Need to be there by 12:30. Takes about an hour to get there, means, I need to leave my house by 11. And I usually have a check list written out several days before of everything I need to bring. I like to be thorough.