This is important to bear in mind, because any claim that impacts the administration of an Estate can have wide implications. It can halt the administration of the Estate whilst the issue is resolved and this can take a long time, dealing with numerous different people/organisations to get to the bottom of what has happened.
Overall, the Courts require any formal claims to be made within 6 years from the date of the negligence. In the case of a Will that wasn’t drafted in time, the clock starts running on the date of death (because until then the Testator could have made another Will).
However, in practice, most negligence claims against professionals are resolved long before the matter goes to Court. This is partly because of the reputational damage a public claim could do, but also to save the costs of litigation, which can be significant.