It varies. I have some writer friends (successful as in multiple books of fiction published, creative writing university teaching gigs, etc.) who don't read at all while they're writing others who do the opposite. I, personally, am always writing something and always reading something. I do need to be careful at times because there are writers with really strong voices who can overpower mine if I'm not conscientious (Kurt Vonnegut and J. D. Salinger have had that effect in the past).
I do keep lists of words and phrases, largely things that it occurs to me that this is the way to describe something I've been having trouble describing, but most of the time I'll keep working at a sentence until it says what I want the way I want it to.
But reading is absolutely essential, and reading to really understand the mechanics of how something works. Francine Prose's Reading Like a Writer is a good introduction to how to do this.