The length of time that seedlings should stay in a seed starter tray varies depending on the specific plants and their growth rate. Generally speaking, seedlings can remain in the tray until they have developed at least one or two sets of true leaves. These are the leaves that appear after the initial seed leaves. Also, you can usually wait until they have a sturdy stem. The root system should be well-developed yet not overly crowded in the cell. Transplanting seedlings at the right stage of development can help ensure that they successfully adapt to their new environment. From there, they can continue growing.