The modern NiCd can be good for up to 12 years of use. (that is an exceptionally good pack with excelent maintenance...) Average will be closer to 4 years service.

To maximize battery life, you need to ensure the pack is discharged and recharged at least monthly. This means that the people who don't fly for 6 months each year need to cycle the batteries durring the non-flying season. A NiCd which is not cycled for 6 months will often go bad.

If you have a cycler and can test the capacity of a battery when its new.. write the tested value and date on the pack with permanent marker. If a cycle test shows it has lost 15% of capacity don't trust the battery. There's about 2 charges difference between 15% loss and 30% loss... then another charge or two and its dead.
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How many flights on a charge? :::: What plane? What engine? What type of flying are you doing?

You can get over 2 1/2 hours of flying of a 2 meter glider using a 500 maH pack if not doing much aerobatics. You can get about 45 min of average .40 size trainer flying from a 600 maH pack. Under adverse conditions, you can get as little as 7 min flying out of a 1200 maH pack flying a 1.20 powered Pattern Competiton aircraft. Depending on how I fly it, my .90 powered Fokker Dr1 can get from 10 min to 90 min per charge from the 700 mah pack in it.

You REALLY need to have a meter to test the pack at the field and see if its safe to take off for the second flight, no matter what you fly.