It depends on the dog. Sometimes ears are fully erect as early as 6 weeks. It will erect on its own without doing anything. I had a chihuahua who has a fluffy ears until he reached 7 months. Now his ears are fully erect. 

By the way, Chihuahuas' ears stand at different points. Sometime they stand from very early on and other times they stand and fall. Be patient. Most Chihuahuas will have ears that stand. It's fairly rare when they don't. Most Chihuahuas have erect ears by 6 months old though.