It's a constant trade-off that becomes part of your house signature. Some of the best beers I've made were boiled for more than 120 minutes, and the very best beer I ever took part in making spent something like four hours on a propane burner to reach volume (Knock on Wood IIIPA).
Personally, I always shoot for volume rather than timing, but it should be recipe-specific (it's an unfortunate side-effect of stovetop brewing that high-gravity, short-boil, and a good mash efficiency are a "pick two" scenario).