When we prepare for GMAT Quant(itative), most of us, if not all, will devise our own "rules" in answering maths questions. Then again, GMAT is pretty mechanical, so to speak. One rule that I find useful and which others will tell you the same is what I call a "2-4-6" rule.
So, what's "2-4-6"? 2 minutes for each Quant question (37 questions over 75 mins, ~2 mins per question), 4 steps for "easy" questions and 6 steps for "hard" questions. Now, what's "easy" and "hard" question? I can't say for sure because it is up to each individual. Some of us have weak spots in certain areas say, geometry but it would be a strong spot for others. "Easy" or "hard" becomes a relative measure. With sufficient practice, your "hard" questions can be "easy".
You must be asking how do I solve an "easy" or a "hard" question in 4 and 6 steps respectively. Well, you need to. 2 mins is not a "long" time. Before you know it, reading and thinking about the question alone will take you the first 30 seconds. Searching for the answer, clicking the radio button and press "Next" will take you the last 30 seconds. That leaves you with ~60 seconds to work out the answer. If you can't do a 4- or 6- step within that 60 seconds, then you know you started on a wrong footing - misinterpreted the question, applied the whole rule(s) or formula, or forgotten the basics.
So, to apply "2-4-6" Rule, it boils down to knowing your fundamentals. As simple as A-B-C.