I found a bottle of lyophilized cycloheximide in the lab refrigerator (4℃). I think lyophilized form should be rather stable, but it's been open for 3.5 years. Cell signaling says the lyophilized cycloheximide can be stored for 24 months at -20℃ (https://media.cellsignal.com/pdf/2112.pdf) and Adooq bioscience says 36 months (https://www.adooq.com/cycloheximide-actidione.html). I don't know if the cycloheximide I found can still be used...