The cart content never expires. It is tied to a customer's browser and will be cleared only if a customer clears his/her browser's local storage/cookies. It was implemented intentionally to increase sales of all Ecwid merchants. When a customer adds something to bag and proceeds to "Your Shopping Bag" page, there is "Your cart contents are stored here for 30+ days so you can return anytime" message displayed, so the customer can feel safe about the things already added to cart, but not purchased yet. He/she can come back to your store later and finish the purchase.
Are the products, added to bag by one customer, available to the others?
A product added to cart doesn't affect the global store's inventory. It affects the look of the store for this particular customer only. The global inventory is changed only when the customer places his/her order. Thus if one customer adds something to cart, all the other customers will see the the same number of items in stock for this product as you really have in your store at that moment.
Your store sells just 1 item. A customer opens your store and adds the item to cart. This customer will see 1 product in the cart and the "out of stock" message in the product details. However, if another customer opens the same store at this very moment, he will see that product is in stock. If you sell a limited product, many customers can add it to their carts simultaneously. However, only one customer will be able to purchase it.