Stability Testing of toothpaste product

I have developed some prototypes I am happy with and will begin stability testing in the coming month. I have a written a stability testing protocol which I will be following including holding multiple samples at 4C, 25C with light exposure, 25C without light exposure, 35C and 45C as well as a freeze-thaw test. I am intending to do USP 51 testing on the products as well as Plate Counts at an external laboratory.

When writing up the procedure I was wondering the best time to do the USP 51 test? initially, I thought that I should do the test when the prototypes are first made however upon reflection it would make more sense to test the 45C samples after the 8 week period, as this would represent a challenge test at the end of a specified products shelf life. Thus passing the preservative challenge test at that time would imply microbial stability for the duration of the products life?

When are USP51 tests done during stability testing? 
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