Alternator belts are non critical to the running of your engine. With most engines, the cam belt is. If your alternator belt fails, you have about half an hour left to get to a safe place. If your cam belt fails, usually you end up on the side of whatever road you just happen to be on with a lunched engine. Saying that though, common sense comes into play here. if you live in a hot country, belts deteriorate faster, so you they need their changing times revising. If you have recently changed it, does it need changing again? No matter how old or how many miles it has done, has it slipped a tooth? The answers are endless with no definative one.
Use of the grey matter will tell you if it requires changing or not. The notes on the boxes are a 'get out of jail free card', and are only recomendations.