These may vary on a case by case basis. A Will does not take affect until the testator has died. Claims contesting the validity of the Will should where possible be brought before a Grant of Probate is obtained by the executors.
Generally, a claim by a beneficiary against the Estate must be brought within 12 years of death (except in a case of fraud).
Claims under the Inheritance (Provision for Family and Dependants) Act 1975 must be issued within 6 months of the Grant of Probate.