You should have your jewellery checked every 6 months or immediately after you suspect it has been damaged.
Sometimes a chain will get caught on your clothing, etc. and pulled., even though you barely notice it. You can tell if it has been stretched by simply measuring the length of it . If you bought an 18″ chain and all of a sudden it is longer, the links will have been stretched, compromising the strength of each one. It may only take a tiny tug on the chain to finally cause the links to pull apart and the chain to fall off.
Rings can be damaged in many ways (i.e. catching them on shopping carts, hitting the sides of cupboards, catching them on pockets, etc. while reaching inside). Sometimes they even get dropped to the floor or stepped on. If this happens and you notice that your ring fits differently, is catching on your clothes or the gemstone seems different in any way, you should take it in to a jeweller to have it looked at. It is much easier and less costly to repair your ring before the potential loss of a gemstone due to its setting being misaligned.