The party itself has no term limits. He can be president for the rest of his life in the party. 
As for the country, he needs a two-thirds majority vote to change the constitution which the ANC does not have (it has 62%, 5% short). It will need another party or parties to make up the difference. That is unlikely unless he can make a deal with the EFF (6%) that they (or rather Malema) cannot refuse