You do not schedule a timeout on each buffered write. You only schedule one timeout on the transition from empty to non-empty that is retired either when the timeout occurs or when you threshold flush (you may choose to not clear on threshold flush if timeout management is expensive). So, you program at most one timeout per timeout duration/threshold flush.

The point is to guarantee data gets flushed promptly which only fails when not enough data gets buffered. The timeout is a fallback to bound the flush latency.