Almost all carpets that are not of the glue-down variety will eventually stretch and as this occurs ripples will begin to form. These ripples will gradually grow larger and become a tripping risk, as well as a generally ugly sight to behold.

How long it takes for this to happen depends on the quality of the carpet, the amount of traffic over the carpet and how well it was installed. These days new carpets are usually layed without the use of power stretchers - this means that the carpets are not layed with anywhere near the tension they used to be when power stretchers were the norm. Result: Ripples develop much earlier than they used to in the "old days".

Wherever possible, re-stretching should be done using a Power Stretcher - whilst a more time consuming technique this method is a long-term solution. Knee kicking is a short-term solution because it cannot produce sufficient tension in the re-stretched carpet to prevent re-rippling for anything but the short-term. 

If left too long the ripples will eventually fold over. This is when permanent damage begins and the backing begins to lose it's strength making it less able to take the strain of re-stretching. Even so, it is often possible to re-stretch folded carpet if it hasn't lost too much integrity.

Having said that, you should always aim to have carpets stretched BEFORE any ripples fold over.

Re-stretching can add many years to the life of your carpet and is SO MUCH CHEAPER than buying new carpets!