Amir al-Sha'bi (may Allah have mercy on him) says that Sayyiduna Umar (may Allah be pleased with him) fixed a term of three years for the full blood money, two years for two-thirds of the blood money, two years for half, and one year for a third of the blood money. And Imam Malik (may Allah have mercy on him) also said the same, (saying) that there are two years for the half because it exceeds a third.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16390