Despite extensive research on task planning in L2 writing contexts, the optimal length of pre-task planning time remains uncertain due to varying lengths used in TBLT studies. This study aimed to address this gap by investigating the effects of different pre-task planning times (nil, 5, 10, and 15 minutes) on the argumentative writing task performance of 80 L2 learners. Additionally, the study explored L2 learners' perceptions of pre-task planning time lengths through semi-structured interviews, given the lack of evidence on this topic. The quantitative analyses indicated that pre-task planning supported Kellogg's Overload Hypothesis and facilitated the quantity and quality of written outputs. In particular, a 10-minute planning time showed the most significant improvement in complexity, accuracy, and fluency. However, longer planning times (15 minutes) led to greater complexity, and shorter planning times (5 minutes) resulted in greater accuracy. The qualitative analyses revealed that while L2 learners had different perceptions of pre-task planning time lengths, the majority found a 10-minute planning time to be the most effective. This study has theoretical and pedagogical implications for TBLT researchers and L2 writing practitioners.