Under Workplace Safety and Health (First-Aid) Regulations section 4, the company is required to maintain the first aid boxes to ensure that they are adequately equipped and usable. 

Although the company is required under the regulations to check the first aid box frequently, there are no requirements regarding how often they are required to inspect the first aid boxes. 

The inspection frequency is up to the company to decide.  

An acceptable industrial practice is once a month unless the manufacturer states otherwise. 

Quoted from Workplace Safety and Health (First-Aid) Regulations