The statute of limitations starts to run after the underlying litigation, including all appeals, is concluded.
This holding is somewhat counter-intuitive; with most torts, the statute of limitations starts to run when the tortious conduct is committed. In the case of legal malpractice arising from negligently handled litigation, the tortious conduct itself does not immediately trigger the running of the statute of limitations.
That is because of two principal factors: First, until the litigation ends, the nature of the damages, if any, flowing from the tortious conduct is not known. Secondly, if the statute were deemed to begin running at the time of the commission of the negligent conduct, it would force the legal malpractice plaintiff to take potentially conflicting positions in the underlying case--which that party presumably would still be trying to win--and the legal malpractice case, where the plaintiff would be forced to argue that his underlying case had been lost or damaged by the negligence. Indeed, the positions that such a legal malpractice plaintiff would be forced to take in the malpractice case could be used against him in the case giving rise to the malpractice claim.
In order to avoid that unfairness--and also in order to avoid forcing litigation to occur that subsequent events might prove to be unnecessary--the courts have settled on the rule that the statute of limitations in such cases does not begin to run until the underlying litigation, including all appeals, are over. See, Apex Towing Co. v. Tolin, 41 S.W.3d 118 (Tex. 2001); Sanchez v. Hastings, 898 S.W. 2d 287 (Tex. 1995); and Hughes v. Mahaney & Higgins, 821 S.W. 2d 154, 157 (Tex. 1991).
It might also be noted that this is one of the few areas of Texas law, over the last 20 years, where there has been a more pro-plaintiff interpretation of limitations law.