The lettuce is growing slowly. As soon as the first tiny "true leaves" started to appear, that was about when the plants were big enough to sway a bit in the breeze when the fan is on. By now they are actually looking like real lettuce plants. I have a couple photos here, taken a week apart.
My lettuce sprouts, on May 7.
The lettuce seeds I got were a blend, so I'm not too alarmed that some of the plants are growing much faster than others; they may be different kinds that grow at different speeds. Also, some of the dripper ends are a little more generous than others, so each set of plants may be getting different amounts of nutrients. Though some of the larger ones do look as though they'd be happier on a space station; gravity is giving them a bit of a hard time here on Earth.
Lettuce on May 14.
You really have to keep an eye on the water levels; the plants get very thirsty as they grow. So, again, it's rather like having an aquarium: if you go on vacation for a couple days, fill it up with extra water before you leave, and if you'll be out longer than that, get someone to watch it for you.
During this phase, the fan and dripper often come on during the morning. On weekends, I see the grow light come on for several hours in the middle of the day, so it must be doing that on weekdays as well, when I'm at work. Some of my coworkers have suggested that I set up a webcam. That would be fun, though a more useful thing would be a sensor for the water tank so I could get actual alerts when it needs more water. There is space to feed a thin hose in for automated refilling, so one could set up something like that, but not to rely on all the time, because the tank should generally be cleaned whenever it gets refilled.