With each brake job.
When ever a brake job is done.
Well to begin with how often do you use your trailer? I recommend that you should check and grease your bearings every six months at the least.
DailyDaily
monthly
Commercially speaking, in the UK they have to be checked annually, and signed.
brake linings should be checked at 15,000 and 25,000 miles and then every 5,000 miles after that.
Every time it is worn!
Once a week.
every 4 weeks in the community
Every time it is worn.
Every time the PFD is worn
As often as they wear out!