More than a few minutes? Seems that way. Makes development rather slow if s.
In general, theme changes will pull down from Dropbox within a few minutes. These changes are saved in "Draft (Preview) Mode" which won't show up on the Live Site until you Publish the changes. If you are looking to see the changes prior to publishing on the Live Site, I would recommend using the "Preview and Publish" tool by going to Website > Click on the name of your website > Theme > Preview and Publish.
If you continue to have delays in changes showing up in NationBuilder, feel free to add a comment below or reach out to us at firstname.lastname@example.org
Complete and current information on how to use NationBuilder is available in the documentation section.