Gates open 2 hours before the game starts. Get to the park early, hang out at Murphy's or somewhere that you can see the line, and you should be able to tell when to hop in. For the Cards game a few Saturdays ago, I was in line about 45 mins before the gates opened, the line was already starting to wrap around Wrigley, but we had great seats. Other times there have hardly been lines at all and I have gone in after the gates opened, and still been able to get great seats. There really is no set time, IMO. Just make sure you are there a little before the gates open in case the line is getting long. There is a ton of fun to be had while waiting if there is no line.