Hindu Media Wiki

In the case of the Ramayana, the exile was 14 years so that Bharata would have enough time to grow close to the people, since previously Rama was beloved by the people and Bharata had been living away from Ayodhya with his maternal uncle; here in Ayodhya Kanda, chaper-9 is what Manthara tells Kaikeyii 

Hence, you have to stop, by force, the arrangements being made for Rama's coronation. You ask your husband as two boons, anointing Bharata for princely kingdom and sending Rama to exile for fourteen years. If you send Rama to forest for fourteen years, your son Bharata will get intimate association in the hearts of the people and will get stabilised in kingdom.

Hence, you have to stop, by force, the arrangements being made for Rama's coronation. You ask your husband as two boons, anointing Bharata for princely kingdom and sending Rama to exile for fourteen years. If you send Rama to forest for fourteen years, your son Bharata will get intimate association in the hearts of the people and will get stabilised in kingdom.

Now as far as the Mahabharata goes, Shakuni provides no justification for why he chooses this particular stakes 12 years plus 1 year incognito for the game of dice; this is all he says about the stakes in the Sabha Parva of the Mahabharata: