How long does brand registry take?

About 1-2 weeks after submitting the application.

It can take up to a year for the brand to be actually approved by USPTO but you will have an application number that you can submit to amazon to get access very quickly (usually about 1-2 weeks for access)

We recommend to apply once you are selling - because its cheaper and an easier application if you have already sold a product under that brand. 

To sell something before its on amazon you can get creative and sell to family/friends, or on your own shopify website.