Your saved shopping bag items will be kept for as long as the product is available. However, these items are not placed on hold and someone else may purchase the item even if it is in your cart.
In the same way, when you place items in your bag but don't order them, they aren't reserved or placed on-hold. Someone else can still order these items even if they're in your cart.
If this happens, you'll be alerted at the cart page that the product is out of stock. To keep this from happening, hurry and snap them up!