Model snapshots are saved to disk periodically.
By default, this is occurs approximately every 3 hours to 4 hours and is
configurable with the background_persist_interval property.
By default, model snapshots are retained for one day (twenty-four hours). You
can change this behavior by updating the model_snapshot_retention_days for the
job. When choosing a new value, consider the following:
A model snapshot resource has the following properties:
description
job_id
min_version
latest_record_time_stamp
latest_result_time_stamp
model_size_stats
retain
model_snapshot_retention_days.
  However, this snapshot will be deleted when the job is deleted.
  The default value is false.
snapshot_id
snapshot_doc_count
timestamp

All of these properties are informational with the exception of
description and retain.
The model_size_stats object has the following properties:
bucket_allocation_failures_count
job_id
log_time
model_size_stats were recorded, according to
  server-time.
memory_status
  (string) The status of the memory in relation to its model_memory_limit.
  Contains one of the following values.
ok
soft_limit
hard_limit
model_bytes
result_type
timestamp
model_size_stats were recorded, according to the bucket timestamp of the data.
total_by_field_count
total_over_field_count
total_partition_field_count

All of these properties are informational; you cannot change their values.