In the Storage FAQ Section How frequently does WiredTiger write to disk it was mentioned that:

MongoDB configures WiredTiger to create checkpoints (i.e. write the snapshot data to disk) at intervals of 60 seconds or 2 gigabytes of journal data.

However as of 3.5.9 this 2GB threshold was removed in SERVER-29210. The 2GB mentioned in the documentation should also be removed.