While "conventional wisdom" calls for rebalancing a portfolio periodically, with most suggesting annual rebalancing, there are studies reporting that re-balancing actually hurts your returns, and may even slightly increase volatility. The bottom line is apparently (1) rebalancing increases costs due to increase in transactions (unless transactions have no tax implications as in a 401(k) and have no transaction fees, as may be the case when investing in no-load funds) and may also increase volatility and decrease returns if done too frequently, so it should only be done when your portfolio has moved by more than a set fraction from your target allocations, (2) a bond/stock rebalancing is probably best done quarterly or less frequently, and between asset classes inside bonds or inside stocks, annually or less, and (3) it is difficult (if not outright impossible) to define a single rebalancing scheme that improves returns and decreases volatility in a reliable way for all market conditions and all investing time-frames.

According to http://www.evansonasset.com/index.cfm?Page=3 :

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Let's take a portfolio which is 70% in equities and 30% in fixed income, a 70/30 mix, with six different equity asset classes including growth and value, large and small, Reits, and U.S. and international asset classes. For the period of January, 1975 through December, 2000, monthly rebalancing produced a compound total return of 3923%, quarterly, 3959%, yearly, 3971%, and every other year, 4233%. This period was chosen because it allowed Reits and other equity asset classes to be included that could not be included if the analysis began in 1973. Thus, for this 26 year time-frame, more frequent rebalancing actually reduced long-term returns.
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Probably the best rule of thumb on rebalancing is to look at the overall stock/bond ratio quarterly, since it is the primary determinant of expected returns, and examine individual equity asset classes once a year, or so. Rebalance only when asset classes, and particularly, the equity/fixed ratio, gets out of balance far enough to produce a significant expected difference in returns. In our experience, the need for rebalancing is a function of market and asset class returns, with big market moves like 2003-2007 more likely to move asset classes out of balance.
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Disclaimer: the above is intended for informational purposes only, and should not be taken as investment advice. For advice about your investments you should consult with a financial professional.

A 401K is a type of trust in which you can hold securities. If you are using a four to five year time span for which to hold stocks you might want to roll twenty percent of them over each year. You do not have to watch them every week. You do not need to fiddle with them more than once or twice a year, if you select wisely. General conditions do not change that often.

Once a year is a good time to look at the percentages and see how far they have skewed off the typical allocation % you are looking for. If they have skewed more than 10% then I would reallocate back to the original % allocations you are looking for.