The answer is 15%, according to CNN journalist Christine Romans, author of Smart is the New Rich. "Live on 85 cents of every dollar," she says. Focus on paying off your student loans as quickly as possible. Anything left over should go into your savings.
The top priority is to have six months of living expenses in a savings account; once you've got that, anything else you're saving should go into investments. Get it out from under the mattress or the shoebox in the closet, and let the money work for you.