I an not talking about upgrading. How do you know when to buy new wheels because the other ones have worn out? How do you tell a wheel is worn out? I can always true a wheel back to true. And nothing is dented, but the maintenance is getting a little more than it should be.
At some point the spokes become too fatigued and start snapping.
At some point, with rim brakes, the rim wears through until it's no longer stable.
At some point the bearing races in the hub become too worn.
Of these, only the third reasonably demands that the wheel be replaced -- wheels with worn spokes can be relaced, and rims can be replaced without relacing. But in some cases, especially if more than one of the above problems is looming, it may be more cost-effective to replace the wheel rather than repair it.
And, of course, there's always the case of some sort of actual damage to the wheel -- ripped spoke holes in the flange, bent flange, "taco-ed" wheel where both spokes and rim will need replacing, etc.
There are several reasons why you might need to replace a rim or a wheel.
- Crash or impact damage is the most common. This can range from a tacoed rim, to flat spots or flare impacts. These are usually very visible.
- Spoke fatigue is a mileage issue. As a wheel is ridden, the constant tension and release of stress on the spokes will fatigue the metal until it breaks. You usually see breaks of several spokes in a short period of time. More than 3 spokes in a few weeks, or spokes breaking at the j-bend for little or no apparent cause are signs of this.
- Brake wear is caused by the use of a rim style brake over extended periods of time. You literally wear away enough of the metal on the sidewall of the rim that it becomes too thin to support the pressure of an inflated tire. This is rare, because a rim usually perishes from damage first, but it is dangerous, and should be replaced. There are indicator marks on newer rims to tell you this is happening.
- There is the possibility that the rim has become fatigued, or that there is small, non-visible damage from previous repairs that prevent the rim from being trued and tensioned evenly. This is most commonly the reason a wheel is replaced. When the rim is trued straight, measure the tension on the spokes (only on the drive side if it's a rear wheel) and if the wheel is round, and straight, but you can't make the tension at least reasonably even, then it will always come out of true quickly. This makes the maintenance load too high for most riders. It is a damaged rim, but in the least visible way.
- And last, if there is damage to the bearing races in the hub, then the wheel will roll rough and have more resistance than it should. The hub should be replaced.
Of course, you can replace a hub, spokes, or the rim individually. If you replace the rim, you should replace the spokes. Why risk building fatigued parts into your new rim? And if you replace the hub, unless the wheel is very new, you should build with a new rim and spokes, for similar reasons. If you can do the work yourself, you will find it cost effective to build a wheel by hand, but if not, buying a replacement wheel (unless it is a high end hand built wheel) is usually more cost effective.
Given your description, I think number 4 is your issue. I'd say time to replace or rebuild the wheel.
I hope that helps.