I think you are putting the cart before the horse.
What I recommend you do is to sit down and decide what your asset allocation (AA) should be. In short, how much of your portfolio do you want in various investments (stocks:bonds, but also how much in each sector).
Once you figure that out you can adjust your holdings accordingly. What you are doing here is basically rebalancing to shift your current AA to your target AA. In terms of timing, the easiest way is to rebalance at a pre-described date - for example the first week of every year, or the first week of each fiscal quarter.
hope that helps.