Answer: A rule of thumb is to set seed flats under lights when a third to half of the seeds seem to have germinated. Generally, seeds (all of the same kind) will germinate with a day or two of each other assuming they are planted at about the same depth. If you wait too long the early ones will get leggy and suffer from lack of light.
If you planted lots of different types of seeds in the tray, some will probably take quite a bit longer than others to germinate. Some people cut the large trays into "six-packs" so they can plant different seeds in each six-pack. That way, they can put the germinated seeds under lights, and keep the slower-germinating seeds on the warming map to hasten germination.
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