Assets grow at different rates—which means that an investment portfolio might end up out of line with the allocation you have chosen. For example, some assets may have recently have grown at a much faster rate. To compensate, you should reallocate some of the value of fast-growing assets into assets with slower recent growth, which may now be poised to pick up steam while recent high-performers slow down. Otherwise, you might end up with a portfolio that carries more risk than desired.
Although there’s no official time-line that determines when you should rebalance your portfolio, typically this is at least once a year in conjunction with an annual review.