Use the --timeout flag to change the timeout.
herbie shell --seed 2019100 +o rules:numerics
(FPCore (x)
:name "2isqrt (example 3.6)"
:herbie-target
(/ 1 (+ (* (+ x 1) (sqrt x)) (* x (sqrt (+ x 1)))))
(- (/ 1 (sqrt x)) (/ 1 (sqrt (+ x 1)))))