Recently ELT has witnessed the debate between the proponents of non-interventionist and interventionist views towards SLA. The former, also known as zero option, sees L2 acquisition as an incidental process leaving no role for instruction (Krashen, 1981; Prbhu, 1987), while the latter considers instruction effective and even crucial for some learner types (i.e., adults and EFL learners with insufficient L2 exposure), some grammatical structures, and achieving higher levels of grammatical accuracy (Dekeyser, 2000; Doughty, 2003). Research has shown that instruction does facilitate the process of L2 learning, especially when juxtaposed with high amounts of exposure (Ellis, 2006). Consequently, different versions of form-focused instruction (FFI) are being practiced despite the fashionable meaning-focused instruction (MFI) of the communicative era. Nevertheless, a question still remains: "when is the time to intervene?" The present research targeting this very question compared the efficacy of proactive and reactive FOF on the grammar improvement of 88 Iranian EFL learners at two proficiency levels of beginner and upper-intermediate. The results indicated that regardless of the proficiency level, proactive FOF was more effective on the grammar improvement of the participants when compared to reactive FOF. The researchers strongly believe that in EFL settings like Iran with minimum amount of exposure, planned grammar instruction is a necessity.