When should you start thinking about using a CDN?

hey everyone, i've been thinking about how we serve our static assets lately. right now, we're just sending images, css, and js straight from our main web servers. but as more people use our site, i'm wondering when it actually makes sense, cost and complexity-wise, to get a cdn. are there certain numbers we should look at, like how slow things are loading, how much we're spending on bandwidth, or just how many requests we're getting, that tell us it's time for a cdn? what's your experience with making these kinds of big changes to our setup