Technically speaking, at no time, because usability testing comes BEFORE beta. However, if you didn't do usability testing before this stage, then I'd recommend you to do it right at the beginning to avoid contamination and confound variables such as Hawthorne effect. This is specially true if your product's features are somehow special or representing new unknown scenarios for the user.
This being said, it's true that if you do continuous research, you can do usability testing at every phase or stage, even on gamma, or even after release. But this is not common, and unless you have an endless budget, it's very impractical. That's why usability testing happens at the beginning (right after prototype stage). Just imagine building something and only start usability testing after completion, just to find out you wasted a lot of time for nothing!
Anyways, for reference, remember this: usability testing is about the user being able to use a feature; beta testing is about whether the user is going to use the feature.