In options, maturities that are shorter dated and at the money exhibit the most gamma. As said before this represents the change of delta vs change in stock price. And the longer you wait to rebalance, the more likely it is you'll have a larger imbalance in your deltas. Delta hedging is very much an art and everyone has a different view when one should rebalance and even to what your real delta is. Often the model delta isn't complete to your risk profile. If you start trading options delta neutral, you'll have to worry about some other risks as well (such as change in gamma, etc). It's not difficult to do, but you should be aware of it.