Use the --timeout flag to change the timeout.
--timeout
herbie shell --seed 2019088 (FPCore (a rand) :name "Octave 3.8, oct_fill_randg" (* (- a (/ 1.0 3.0)) (+ 1 (* (/ 1 (sqrt (* 9 (- a (/ 1.0 3.0))))) rand))))