Magento Community Edition, 1.9.x
Quote Lifetime

You can determine how long a price is valid by setting the shopping cart quote lifetime. For example, if a shopper leaves a cart unattended for several days, the price of some items might change. By default, the quote lifetime is set to thirty days.

To configure the quote lifetime:

1. On the Admin menu, select System > Configuration. Then in the panel on the left under Sales, choose Checkout.
2. Click to expand the Shopping Cart section.
3. In the Quote Lifetime (days) field, enter the number of days that a quoted price remains valid.
4. When complete, click the Save Config button.