The portion of your portfolio that should be allocated to equities depends on your ability, willingness and need to take risk. One of the factors affecting the ability to take risk is time horizon: investors with a long time horizon can allocate a greater percentage to equities than investors with a shorter time horizon. Larry Swedroe in Rational Investing in Irrational Times provides the following guidelines for the maximum equity exposure depending on time horizon:

0 to 3 years – 0%
4 years – 10%
5 years – 20%
6 years – 30%
7 years – 40%
8 years – 50%
9 years – 60%
10 years – 70%
11 to 14 years – 80%
15 to 19 years – 90%
20 years or longer – 100%

Note that other factors such as your willingness and need to take risk will determine how much you actually allocate to equities. I found this table interesting because it provides a rough guideline of how much could be allocated to equities.