Depends on the release but a good rule of thumb is an hour early. If there is something special I'd go 2 hrs to guarentee you get what you want.
Generally, to get the one you want, I'd arrive AT LEAST 30 minutes before they open, but honestly, you can pretty much guarantee you get the crowler you want by getting there an hour early.
I would say for Thursday releases (during the summer) if you arrive at 3pm for a 5pm open you'll likely be in the top 10 in line. 4pm would result in the top 50. Usually the first 100 - 150 get crowlers. They do a good job of counting the folks in line and limiting quantities to ensure everyone gets some. With that said, I've also been in line where it was 40 of us total and got to go through the line twice over (it was raining quite a bit that day).
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