What is the inspection frequency that a letting agency will execute?

We're having a string of issues with our properties, some due to normal wear and tear, but others definitely due to tenants negligence or outright destructive. I think that some of the damage could have been avoided had three letting agency conducted inspections from time to time. The agreement with them states that they will do periodic inspection. When I pressed them on what periodic means that said every 12 months.

Are these the usual terms of these agreements?

Thanks in advance.