By law, your plan administrator (employer) must allow you to change your allocation at least quarterly, but most plans allow for more frequent changes.
Generally speaking, you can change your allocations as often as you need to with no commissions or fees; that is, up to a point. Many plans start to impose fees after about the 10th reallocation, and partially this is meant to discourage over-trading.
By law, a plan must permit participants to reallocate at least once a quarter, but most plans accommodate more frequent trading. As a rule of thumb, you should probably not change your allocation more often than once a month.
Some plans may also facilitate automatic rebalancing to your original asset allocation at regular intervals. Some would argue that maintaining an allocation through rebalancing is the surest way to achieve the desired investment results, as per the theories of Nobel Laureate Harry Markowitz.