The time frame will depend on how the users interact with the site. The most downloaded assets will get loaded to the CDN caché, and will be faster if you’re closer to the CDN server than from the server.
If you have the datacenter next to you, and the CDN server is 1000Km away, it will always be slower. The idea of the CDN is to serve better a geographically diverse audience, not to improve centralized visits.