There's no exact time or date that the Supreme online clothing store restocks their merchandise. However, from checking Supreme's official Facebook page, it was found that they often restock on Thursdays, generally once a week.
Conversations about a restock happening in March has been posted on the page. It's likely that they restock heavily in March for the Spring/Summer season.
To find out for certain, you can email them at shop@supremenewyork.com. Or, call the New York store at 212-966-7799, or the Los Angeles store at 323-655-6205.
To get instant notifications on your iPhone or Android when the brands you love release new items, restock or go on sale (supported brands currently include Supreme, Nike, Palace, and more), download the Restocks app.