I think you should replace wheels when the cost of maintaining them becomes higher than the cost of a new set, a very real possibility when dealing with low end stuff. If too many things need doing, it may be more economical to get a new wheel. However, you have to add the cost of truing the new wheel because machine-built wheels can sometimes get out of whack a little quickly and require a true, after which they seem to stay true better.
You should absolutely replace a rim when there are cracks around the spoke holes or the brake surface is significantly worn down.
If spokes start breaking often, you can conclude they're all going to go because they're nearing the end of their fatigue life, unless the wheel was unevenly tensioned. Respoking is too pricey for most low end stuff. I redid a 3sp c/b wheel once, but I can spoke a wheel myself and UCP spokes were still available individually from Niagara for $0.14 apiece.
Multi-speed freewheel wheels are likely to need a cone and axle job every now and then especially with a strong or heavy rider, 7 or more sprockets, or rough use. The design was created for 4-speed freewheels and when they went to 7 the axles started fatiguing much more quickly. Those are worth fixing if you can do the work yourself; otherwise it may be more economical to get another $50 wheel. Get a shop to pull off the old freewheel and just screw it on hand-tight on the new wheel because pedaling will tighten it, and grease the threads preferably with antiseize. I feel this particular fault can depend a great deal on how the wheel was put together and there's no hard rule for how much breakage to expect.
