I made a 5 gallon batch of cyser at the end of january in a glass carboy with a 3 piece airlock. I kept an eye on the fermentation but by the time it was ready to rack I forgot about it -- a couple months later (today) I go to siphon it into secondary and notice that the liquid level in the airlock is below what is needed to keep the air out. It wasn't completely gone but assuming linear evaporation I would guess it was insufficient for about 1 week, maybe 2 at the most. The airlock cap only has these tiny pinholes so it seems somewhat resistant to air infiltration on its own.
The cyser didn't have any unexpected odor.
I know the only way to know for sure is to try it but based on my last batch (a plain mead) it's going to taste bad anyway until it ages longer so I don't think I'll be able to tell if it's a little off at this stage.
Based on FG it's 15% ABV