I decided it was time to stop coddling my little sprouts. At this point, all of my seedlings had a good amount of secondary leaves and seemed ready to be moved out of the grow lights.
I had originally planned on keeping them in the lights for a long time, but changed my mind when I saw how the lights treated two of my older seedlings. I had a two-year-old and a one-year-old under the lights for three weeks. At the end of the period, the leaves had turned a deep blue-green color, and the trunks didn't seem as rigid. The color is an indication that the leaves were adjusting to find the best color to absorb the artificial light.
The color of a leaf shows the light waves that its chlorophyll reflects, and thus, doesn't absorb. Since the leaves were a little more blue than the ones outside, I can deduce that my lights are lacking in (at least) the blue spectrum.
As for why the trunks seemed less rigid - that may be due to there being no wind in my garage, that the light may not be bright enough, or that it's only really reaching the top parts of the tree, giving too little to the tree overall.
Anyway, the tree seedlings have spent two days in full sun with nothing but positive signs. The soil mixture is well-draining enough that I'll have to water these every day, possibly twice when it's really hot outside. I'm going to play it safe and keep the trees close to the house to protect them from heavy wind and rain.
In this photo are my coast redwoods, giant sequoias, and bristlecone pines.