New conjuring stone heroes last about a week once they're introduced, if I remember correctly. It feels like Havoc was only in the stone for a little over half a week though... That may just be my bad memory, however. On the bright side, they brought him back near the beginning of the next month so even if you miss Aqua now he'll likely reappear in the stone fairly quickly.
Server: 090 Spartan Dagger
Guild: Leader of AVALANCHE - 50579