JB=9: "10 minutes plus, depending on pilot weight," says Mayman. "For the technically minded, it also depends a little bit on temperature, altitude and that kind of thing, but that's by no means as significant as the total pilot weight."
Flyboard Air: "and a maximum endurance of 10 minutes. "
Williams X-Jet: "Endurance: 30–45 minutes"
One of these was cancelled by the Army in 1983. The other two are "the future is here."