If you have a small website that only your friends visit, you probably have never considered using a Content Delivery Network to host your images or other files. When your site suddenly exploded with popularity, you might have wished you chose to use a CDN sooner. The question is: When is the right time to start using a CDN?
As with many website decisions, it ultimately comes down to numbers. If you have a very active site and are building links and taking no prisoners, it will reflect in your traffic statistics. Check the bandwidth numbers and how much of that bandwidth is taken up by content you could upload to a CDN. If a CDN will save you money by reducing bandwidth, it is certainly worth considering.
Another motivating factor will be speed. Even if your site is not leaking bandwidth, a CDN may give you a speed boost by providing redundant connections to your content from diverse geographic locations. Although that will not necessarily save you money, it will make your website snappier and more usable for your customers or site visitors.
Combine both of those factors, weigh the pros against the cons, and you should come up with a clear picture of when your site will be ready for a CDN or if it is long overdue. Once you have decided to use a CDN, you can then start the process of choosing the right one to fit your needs, which is another issue entirely.