As a general rule of thumb harder veggies have more fiber and need more time to break down, but in my experience it's really all about how small and into which shapes you cut your stuff. Also kinda depends on what you wanna do really, if you're trying to get color on stuff don't overcrowd your pan, but if you just sorta wanna sauté/steam your stuff, you can add more stuff at once and sorta layer it while keeping it moving.