When is it too late to try to take a voluntary dismissal on your federal law claim in a removed case?

In Miller v. Terramite Corp., the Fourth Circuit in an opinion by Senior Judge Hansen of the Eighth Circuit, joined by Chief Judge Wilkins and Judge Williams, held that by waiting until after the completion of discovery and the filing of defendant's summary judgment to seek dismissal or amendment to drop its federal law claims, the plaintiff had waited too long, and the trial court was within its discretion in denying plaintiff's request to drop the federal question and seek a remand to state court.