The general rule with squirrels is that it's independent if it's nearly full-sized, has a full tail, and can run, jump and climb. However, it's okay to intervene if a baby squirrel falls from a nest or the entire nest falls from the tree.
If you know the baby squirrel (or nest) fell within the last several hours, give the mother squirrel a chance to retrieve it. If the baby or nest isn't reclaimed within a few hours, you may need to take matters into your own hands. Take steps to keep the orphan warm and call a wildlife rehabilitator.