Frequency of Loki writing to backend storage are controlled by three configurations, max_chunk_age, chunk_target_size, and chunk_idle_period. Unless you have a good reason to change them, default should work just fine.
You also shouldn’t care about when you’d see files in S3. It does not matter, you should be see logs from Loki right way once it’s ingested regardless if it’s written to S3 or not.