The more time you have, the more risk you should take. If you're just starting out, 80 percent to 100 percent of your assets ought to be in stocks. The simplest trick? Subtract your age from 120: That's the percentage you should have in stocks; the rest should be in bonds. "If you have, say, 30 or 40 years, what happens over the next three months or even three years doesn't matter. If you need the money in two years and it drops 40 percent in one year, that's a problem," says Stuart Ritter, a certified financial planner with T. Rowe Price.