Wait until the leaves are about the size of a dime.
Fred in nj

Donna, there are two things to know if there are ready. First, are the leaves on them at least the size of a dime, maybe closer to a nickel? Some people feel dime is fine, others prefer nickel size.

I recently read something that said the following, "shape into better defined clusters.
When one picks up the pot and spreads the clusters it should begin to look like a number of tiny leaves are formed from a single stem. If it does not look that way it is too early to cut them apart because cutting into the mass may result in a gob of soil with more than one plant. In most cases some baby plants are cut through the middle because the individual plants have not yet been well defined."

So, I'd say between these two, you've enough info to decide to split them or not. I imagine that they are plenty large enough simply because of the time frame, but plants are different so this info should help you make the decision.