Recently ELT has witnessed the debate between the proponents of non-interventionist and interventionist views towards SLA. 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 120 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 at beginner level when compared to reactive FOF, while at upper-intermediate level there was no significant difference between learners who received proactive instruction of grammar with those who received reactive instruction of grammar. Hence, the researcher strongly suggests that in EFL settings like Iran with minimum amount of L2 exposure, planned grammar instruction is a necessity.