I dont want my plants to become rootbound in solocups. So far it has been 2 weeks in seedling stage. I plan to put them in 3 gallon fabric pots.
When the leaves reach the outer sides
@Oldschool82 nailed it.
I like to wait a bit longer than that.
When my sprouts are sucking the cup dry in 1 day after a full watering is when I transplant. Because at that point, I know they have great roots and will hold the soil together - won’t fall apart on ya which is the worst thing during a transplant. They usually look like this when I transplant.
I would agree this is ideal for photos. Not for autos.
Out of likes @Hellraiser
I do what @Hellraiser said, but a lot of people use a clear cup same size as the “red” cup, and then they can pull the clear cup out and see the roots!