The effective service life of NiCads could be a large and complex complex topic. If you are working only with a single cell or two, it's probably best to replace it/them with new ones which will probably have a greater service capacity as a secondary benefit.
More likely than not you will be offered Nickel Metal Hydride cells as replacements for single cells anyway, and for most purposes they are an effective transplant where NiCads were in previous use. NiMh cells do have a faster self discharge rate when idle than their older cousins, although there are versions around now which claim to have overcome this problem.
Low self-discharge NiMH battery - Wikipedia, the free encyclopedia
I have stretched out the life of NiCads to 8 or more years by various devious practices which I would not recommend for OH and S reasons
. Some respond favourably to this treatment, whilst others clearly have had enough and need to be retired.
If you are using a pack of NiCad cells rather than individuals, a replacement or a repack is probably the only option.
Finally, in some applications, NiCads are still a better service option but it depends on their intended application.
As I said earlier, potentially a long and complex topic
Regards,
FoR