The time listed on epoxies is the working time, not the curing time. Some manufacturers confuse things by putting the cure time on the bottles, but they are in the minority.

30 minute epoxy has a 30 minute working time and a set time of about 4 hours. All epoxies take months to fully cure - the chemical process never really stops, but 24 hours is good for almost any epoxy 5 minute to one hour.