According to Dave Meltzer in the latest Wrestling Observer Newsletter, Brock Lesnar's current contracts expires either in May or June of 2020.
It's said that he last signed a contract with the company last November and that expired in April following WrestleMania. Lesnar then took advantage of the timing of his departure and those UFC return rumours to score a big new deal with WWE, and Dana White has since said that The Beast has now fully retired from the Octagon.
Lesnar returned to WWE television to win the Money in the Bank briefcase last month and failed to successfully cash in on Universal Champion Seth Rollins at Super ShowDown. He hasn't been seen since despite Paul Heyman teasing his return.
Still, with such a lengthy contract, we're bound to see more of The Beast soon enough...