This study investigates the developmental trajectory of filler use by Chinese learners of Japanese. Using Beijing Corpus of Japanese as Second Language (B-JAS), it quantifies changes over four years in role-play tasks (requests and refusals) and compares them with native speaker data from International Corpus of Japanese as a Second Language (I-JAS). Two key findings emerged. First, while the frequency of filler use decreased across academic years in both task types, the rate of decline varied. A steeper reduction was observed in more cognitively demanding refusal scenes. By the fourth year, learners' filler frequency in request scenes approached native-speaker levels, whereas a significant gap remained in refusal scenes. Second, the forms of fillers shifted from simple vowel-type fillers in early stages to more lexicalized forms (e.g., ano and etto), indicating a partial convergence toward native-speaker usage. These findings suggest that the development of filler use among L2 learners is not a linear path toward native speaker norms, but a dynamic, learner-driven process shaped by task-specific cognitive and pragmatic demands.