It depends a lot on your risk tolerance and risk objectives. It can also depend on your level of investment education. Ultimately, it depends very much on what kind of assets you use over time.
All that said, if nothing else, you have access to getting diversified investment exposure with some professional help through mutual funds. A good base for setting expectations for a decent long-term return would be to find a passively managed balanced mutual fund such as one from Vanguard, Fidelity, Schwab or other such financial institutions, and examine its long-term return pattern.
For example, as of March 31, 2013, I looked up several of these funds and over the past 10 to 20 year periods (which we could debate about using, but that is for another conversation), the average annual returns ranged in the 6-8.5 percent range. That’s not too bad, given the 2008 crisis we went through and continue to recover from.
A balanced fund like this would be considered to have middle of the road (“moderate”) risk with moderate return objectives. Hence, if you were using more conservative investments over the same time frame, 4-5 percent may be more reasonable to expect. With a more aggressive approach, 8-10 percent or higher would not be unreasonable.
Because investing is complicated and is becoming more complex over time, we generally recommend that unless you plan to spend considerable time on it each week, you work with a qualified professional to establish and manage your investments. They will not only be able to help you position your portfolio appropriately given your personal risk tolerance and objectives, but they will also help you stick to your long-term plan when the going gets tough.