Boots that are resoleable have soles that were stitched on. If the soles were glued, then they can't be replaced. You can easily tell if a sole was stitched by looking at the edges of the sole for signs of stitch holes.
Sole stitching will eventually wear down, and almost become smooth and invisible. But that's only the outside stitching. There is stitching on the inside of the sole that holds the sole onto the boot foot that isn't damaged from walking, and doesn't “wear” like sole stitching does.
The time to resole boots is when a hole begins to develop on the bottom of the sole, or when the stitching begins to separate on the side of the boot (between the sole and the foot) and thus a hole might open in that area. Or, of course, resoling is needed if the sole got significantly damaged by stepping on a sharp object like glass.
Leather on cowboy boot soles is very tough and durable, and will last a long time. Same is true for soles of motorcycle boots which are often made of a hard compound like rubber (Vibram is a specialized type of rubber.) It can be expensive to resole boots, so you should only do it if there is a definite need to do so, such as for a favorite pair of boots that you want to keep wearing but the soles wore out.
A good cobbler – one who knows his craft – can look at a pair of boots and tell you if resoling is necessary or not. A good one won't recommend resoling unless the boots really need it.
Metal toe and heel caps (also known as rands) can add to the life of a boot sole. However, they make boots noisy, and may not be acceptable in an office setting.
Summary: when to resole boots? If there is damage or holes in the sole, and if a cobbler recommends it. And, ultimately, if the boots are worth it. If the boots were expensive and/or are among your favorites, then having them resoled is worth the price. It will cost about US$50 -$80 to resole boots with a standard leather sole, and more if you choose a specialty sole, like a Vibram lug.