There's a business major in my class, and a few non-science majors (myself included). I took the prereqs at a community college, 2 classes per quarter. It took a year and a half or thereabouts. I took all but 3 of the prereqs this way.
Each quarter I took one night class and usually a day class. During this same time I was able to get a job as a rehab aide, and would shadow at a hospital or outpatient clinic once a week on the side to put together a decent range of experiences. I gave myself roughly 2 months advance to study for the GRE, and I retook it one time to improve my quantitative score (after verifying w/the two universities I applied to that they would either average GRE scores or accept the most recent one).
Specifically regarding the prereqs, my advice is to not bite off more than you can chew in terms of taking more prereqs at a time than you can handle. The priority should be basically getting straight A's on them in my opinion. Two classes/quarter was about right for me, although another guy I know took one per quarter even though it took him a lot more time. And of course, if you know you can handle 3/quarter than by all means minimize the time if it will save you a year in applying.
There's a lot of students at community colleges who are perpetually retaking courses to try to improve their scores. Save yourself valuable time and money by staying focused.