Depends on your area. We have 6 Apple stores in D/FW, lots of AT&T stores, lots of BB, RadioShacks, and Wal-Marts. I suspect a lot of the opening day line will get dispersed a bit.
For the 3G I got to the store about an hour before opening. Due to some difficulties w/ my FAN account, the entire affair was about 2 1/2 hours IIRC.
Got there about the same time for the 3Gs. I think the crowd was slightly smaller, and I had no issues with my FAN account. Was in and out in about 1 1/2 hours. (incl. the one hour wait before opening).
So, you need to see if your friends have done it before and find out what kind of lines you can expect. Best bet, if you work @ 4PM...go really early and camp out. Get your phone, play a bit, go to sleep...get up for work.