After A Reasonable Time
If a fault appears after that nebulous "reasonable time" has elapsed, don't despair. If it's in the first six months, and it's not because of fair wear and tear, accidental damage or misuse, then the retailer must still repair or replace the item. If he demurs, he has to prove the item wasn't faulty in the first place, or that it couldn't be expected to last that long. Often he'll choose to replace rather than repair it.
If more than six months have gone by, things change. You might still get a repair or replacement, but now you'll have to prove that the item was faulty when you bought it if the retailer disputes your claim.