After seeing so many questions in general about the minimum/suggested time to incubate annual killifish eggs in peat, I got curious about the opposite:
What is the longest that you've incubated eggs out of water and still hatched out viable fry?

This varies from species to species.

The most impressive for me was a packet of N. guentheri peat which was 6 months old and bone dry. I wouldn't have bothered to wet the peat if I didn't see eyed-up eggs in it.

N. furzeri regularly go 9 to 12 months.

The most impressive story I've heard involves a packet of Pronothobranchiys seymouri that laid behind a shelf in a fishroom for 26 years and yielded viable fry.

I have a packet of korthausae eggs that is now about 9 months in incubation. I kept it in my office draw (66-68 oF). No sign of development. Same for some cardinalis eggs which are now about 6 months old. If you keep the eggs below 68 oF they seem to go into diapause I and remain there. For how long, I don't know... yet. The critical thing is to make sure they don't dry out. That is what was so amazing about the guentheri eggs. The peat was bone dry.