My daughter had one time an asthma attack, for how many months should she take singulair (montelukast) and steroid inhaler?

3-6 months. If she is on both singular and a steroid inhaler, it sounds like she has had more than one asthma attack in her lifetime to be put on two preventative medicines. Normally we'd like to see good control for at least 3-6 months before we start weaning preventative drugs. .