Once a year is plenty.
The priority should be on “rebalancing” your mix of stocks and bonds (known as your “asset allocation”). Not only does the ideal mix change as you age, but the balance can get out of whack when certain asset classes do better or worse than others. (After a big stock market run, for example, the percentage of your asset in stocks is probably too large.) What you need to do is take the money out of the overgrown asset class and reinvest them into the other.
One big benefit of rebalancing like this on a regular basis is that it forces you to buy low and sell high.