After a couple of years if your laptop is still
Most people tend to treat their laptop pretty well by keeping it protected in a carry case and just generally looking after it. The same can't be said for chargers sometimes as they get dropped and lost - and it's not until you're without a laptop charger that you realise just how inconvenient it is. Of course if your laptop has no power supply, as soon as the battery runs out you can't use it, and that's when you might need to start looking for a replacement laptop adapter.
Sometimes you may see your charger becoming faulty, and you may be able to buy a new one just before it gives up the ghost. This isn't always possible though, so there may be a very short period of time in which you're left not able to use your laptop, whilst you wait for your new charger to arrive. In times like these, be sure to order a new one quickly from a reputable firm who can offer you a fast dispatch.
It may work out cheaper for you to buy a second hand charger sometimes, but this probably isn't the best route to take. With as second hand one you've got no way of knowing how much use it has had, how many times it has been dropped or damaged, and ultimately how long it will last you. For extra peace of mind it's a good idea to go with a brand new replacement laptop charger - where possible a genuine product manufactured by the firm who made your laptop. It's not essential that you go for a genuine replacement, but it is advised. That said, there are some decent replacement laptop charger brands out there who offer good products at great prices.
Don't bury your head in the sand if you see your charger is becoming faulty, address the problem. You never know when your charger might pack in, and if you've been given some forewarning, it's always good to plan for the worst. You never know when you might need to access your laptop to retrieve an important file or piece of work - if your laptop is inaccessible because your charger has broken, this is obviously going to pose a problem.
Laptop adapters may need replacing at some point during the life of your laptop, do your best to shop about and see who's offering the best deal to ensure that you don't pay over the odds for your replacement.