This depends a lot on temperatute and other intrinsic factors that cannot be assayed except by experiment.
N. luekei eggs can go about 7 months if not more in peat in and then you will still find un-eyed up eggs. How long the eyed-up eggs last I am not sure but some of my eggs the last time I wet them never hatched, presumable because the fry was too old and weak to break free of the eggs.
In the wild N. orthonotus, furzeri and rachovii may go several years without hatching because there was no rain. These eggs probably do no exist in the wild as eyed-up eggs though. Marc Bellemans found only clear undeveloped eggs when collected eggs of N. virgatus and sp. aff. rubroreticulatus from mud in Sudan.
For eyed-up eggs there seems a very short window within to get good hatches. For N. sp. Mansa and symoensi this is only a week or two. For rachovii it may be a month or two but this depends a lot on temperature.
The higher the temperature the higher the maintenance cost of tissues and so the resting fry has to use its energy reserves to repair damage. Once the yolk is gone it will begin to metabolize its tissues... from then it only a matter of time before it has wasted away in the egg.
If you have eyed-up eggs it is best to wet them ASAP IMHO.
Regards
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