Actually there are no set rules. It is up to the individual and, in fact, in most cases there is very little activity once the allocation is set up for the index investor. For those who have a need to be in and out of the market, I suggest a small trading account on the side.
Let's take a look at an actual allocation to see what we're talking about:
Most people would want to follow their asset allocation for all of their assets combined, including brokerage accounts, IRAs, 401(k)s, etc. You can do this the old school way, using an Excel spread sheet, or with online services like Personal Capital that collect account information from various sources.
In looking at the Schwab allocation above, you want to eyeball the "Difference" column. This tells you when to rebalance. Note that "Fixed Income, in this example, is under allocated by 2%. If this is your rule (i.e., to rebalance at 2% difference), then the sector needs to be bolstered by 2%. As it turns out in this case, this increase can be met by reducing "Cash Investments" which happen to be over invested by 1.8%.
You probably have noticed the "View Table in $" link at the top of the table. If you click, you'll see the dollar amount to be invested. Divide by the fund to be purchased and you have the number of shares to buy. It's that simple. Inside of 5 minutes, you are done.
Keep in mind that, if you are using commission-free funds (most major providers today - Fidelity, Schwab, Vanguard etc.- offer them ), the number of shares to be bought or sold doesn't matter. If the commission is approximately $10/trade, then you want to buy sufficient size so that commissions are not having a material impact.