Most people will agree that a dog reaches his or her prime hunting performance at 4 years of age. Some reach it sooner, some later. Lot has to be said for breed, bloodlines, genetics, training, and dog's natural ability. How long that prime ability lasts may be determined by how well we take care of the dog, and how hard we hunt them. So over time dogs will usually lose the ability to hunt, some may lose the desire. Wish I had a formula to determine this number, wouldn't have to make my living fixing plumbing problems dealing with other people's crap. Be interesting to hear from others how old their dogs were when they reached the point they could no longer hunt.(Sounds like a new thread starting up, get right on it).
Already have a few numbers out there for you as to when to acquire your next partner. I made a pact with my first shorthair in '94; "we were never gonna get sick, we were never gonna get old, and, we were never gonna die." She lived to be 16. Do the math, every 5 years would have made us a 4 dog household. Now if your spouse wants one of her own to always have a dog at home while you and "your" field companions are out tearing up the countryside...
Don't forget to take into account your current dogs personality. Not all dogs get along well with others. Some households can only be a one-dog-household. Not all perform well "competing" with others. Seen one dog that hunted great by itself, but wouldn't while hunting with others. The agressive habits, busting points, stealing retrives, can be corrected with training. Haven't figured out to correct the regressive behavior.
Lets also take into consideration your ability to divert time away from family, job, current dog, and direct it towards training your new partner. All things considered, your timing for your next hunting partner is entirely based on what is right for you, your family, and your hunting habits.