18 months - 2 years would be an appropriate time to use ES6 features safely without a transpiler (however we will still be using transpilers in the future for E

18 months - 2 years would be an appropriate time to use ES6 features safely without a transpiler (however we will still be using transpilers in the future for ES7 and ES8). But you can use them today using a transpiler like 6to5 or Traceur.