What 

What is the Preserved Content Update job and how often should it run?

How 

It is important that the Preserved Content Update job runs every minute.

• Storing files in temp folder 
  When a file is uploaded into Pure, the file is placed in a temporary folder (temp folder), where only the user who uploaded the file can see it (and only while the user is still logged into Pure). 
  This is to insure, that if a user uploads a file, and then regrets and remove the file again before saving it, no one else should be able to see the file in Pure. 
• Storing files in perm folder 
  When the Preserved Content Update job then runs, the file is moved from the temp folder to a permanent location* (perm folder). 
  *If Pure is configured to use external storage the job will be moving/copying content over to the external storage, this being the permanent location.    
  The file access rights are now changed, so other users can now see the file as well. 
  Because of this logic, it's important to run the job regularly.
• Preserved Content Update job performance and logs  
  Because the job is expected to run every minute, it does not create a log file for every run, as the volume would be too massive. Therefore it only creates one log file once a day (at night), this is to avoid that we fill the database with a lot of 'everything is okay'-logs. Note, if the job encounters serious bugs it will create more logs. 
  On each run the job will only check/update files that has been changed since the last run, so you should not fear for any performance issues. Running this job that frequently does not stress your system - when running that often, it will be quick. 
  Do note, that once every 24 hours (at the first run after 1 am) the job will do a full scan of all files, to see if there are any problems with access to the files.
  • If the job does not create a daily log 
    If you have only scheduled the job to run once per day instead of once every minute, running this job with so low frequency causes it sometimes to miss generating a log file as this job is designed to run highly frequent, but only generate 1 log per day. If you re-schedule it to run once a minute, it will generate a log file every day, and thereby you'll see the log become more consistent compared to former schedule.

Note: Make sure not to mixup the 'Preserved Content Update job' with the almost identical-named 'Preserved Content Maintenance' job, when it comes to scheduling.   
The 'Preserved Content Maintenance' job should only run once a day or less, since this job is checking if all content in Pure is okay, no matter if it's updated or not, so it puts a strain on the disks and database. 

If this job is running with less frequency, users might experience this scenario: https://helpcenter.pure.elsevier.com/pure-core/why-cant-i-see-the-files-after-changing-publication-template  

More information

Pure Master Class: Housekeeping - Keeping Pure Healthy (Part 2) Pure Academy video (Brighttalk)