The main purpose for stability testing is to establish a product profile to ensure that the product is within the set chemical, physical, microbial and container closure parameters.
As per the stability guidelines the recommended testing frequencies are as mentioned.
For long term studies, frequency of testing should be sufficient to establish the stability profile of the drug substance. For drug substances with a proposed re-test period of at least 12 months, the frequency of testing at the long term storage condition should normally be every 3 months over the first year, every 6 months over the second year, and annually thereafter through the proposed re-test period.
These frequencies are to ensure that the proposed shelf life is actual/ real and applicable for the product to sustain the determined or to be determined shelf life.
Hope this clarifies your query