The default configuration is to replicate synchronously and persisted to disk asynchronously. So this means any writes are replicated on another host before the transaction is acked as committed while at the same time being written to disk asynchronously as fast as the disk can write them out. So there isn’t a precise frequency of writing to the disk in async durability. Unless there is a huge burst in writes, the disk writes happen right away - within a few ms of commit (or even before the commit is acked). If disk writes do get far enough behind the workload (so there is a build up of data not written to the log) we will eventually throttle commits to allow the disk to catch up.
If you want commits to also flush to disk, then you can create your database with sync durability.
CREATE DATABASE XXXX WITH SYNC DURABILITY
The performance overhead of sync durability in most cases is not very noticeable (10-20% - its very workload dependent).