Modern cloth diapers often contain elastic around the waist and leg openings to allow for a snug fit and prevent leaks. If the elastic begins to wear out, the diaper will not function properly. The elastic in cloth diapers should last for many years (and through multiple children), however mistreating diapers can cause the elastic to wear out quicker.
Three tips to avoid elastic wear and tear:
1. Do not use additives such as vinegar. Vinegar can seriously mess with the pH of your diapers and may lead to diaper rash, but it can also break down elastic fibers and waterproofing in cloth diapers.
2. Don't stuff hot pocket diapers right out of the dryer. Hot elastic right out of the dryer has more stretch than elastic at room temperature. Stuffing hot pocket diapers can cause the elastic to stretch-out quicker. Let diaper covers come to room temperature before stuffing them.
3. Avoid homemade detergents. I love DIY projects, but chemical composition and changes in pH are difficult to control in a home setting. A homemade detergent that is too acidic or basic can seriously damage PUL, elastic, and even the metal parts of your washing machine. I recommend Rockin' Green detergent or FuzziBunz detergent (by Rockin' Green) because the clear-rinse formula is specifically designed for use with diapers. Commercial cloth diapering detergents have very low "price mark-up", and DIY detergents are expensive enough not to justify this as a DIY project.
Replacing elastic is very simple. Some of the major diaper manufactures sell kits to refurbish diaper elastic (BumGenius) or simply provide extra elastic in the event you need to replace it (FuzziBunz).
Using a BumGenius refurbishing kit:
How to replace the elastic in a FuzziBunz Elite diaper:
In my experience, elastic that is well cared for should not need to be replaced for many years. I've occasionally heard of people replacing elastic, but it's usually after the diapers have been handed down through multiple children (or multiple families).