It’s all down to your judgement really and that comes with experience. And as Martin says, after years of doing 3D, sometimes you misjudge how many verts you need and you add too much or too few. Just give it time and patience and in time your brain will start to correctly estimate, more often than not, the amount you need for what your working on. The important thing is to notice a mistake in time and not being scared of starting over, because one tends to want to fix the problem and you often loose more time than if you just start again.
