There is no hard and fast rule -- especially in your situation. The closer you get to retirement, the more you want to minimize your volatility risk and start moving towards bonds and cash equivalents. However, these types of investments have lower rates of return, so they aren't going to help you to "catch-up" for a savings shortfall. If you are behind in your savings goal, then you may want to be more aggressive with your investment. This, however, will give you a lot of exposure to the possibility that your investments could decline in value.
You might want to sit down with a financial planner. Places like Charles Schwab will talk with you generally if you have accounts with them. They aren't being paid on placement. Your advice will be more generic, but they shouldn't be pushing you toward high fee investments. If you want more customized advice, you can look to hire a CFA who can go over your plan. You'll want to find one that works on a fee basis from you, not from the investments you buy.
Somethings you'll want to look at are:
1. What type of expenses you expect in retirement
2. How much you have saved now
3. Whether you have to retire in 13 years or just want to (if it is a desire rather than need then you might be able to be a bit more aggressive in your asset allocation and live with the risk that you might need to work an extra 5 or 10 years if your investments decline)
Good luck!