Longboards are developed to last for several years. The quantity of time you use your board will rely on the maintenance and care that you give it.
When you have your longboard and used it for a while, you will notice that the wheels are starting to get looser, or that the wheels on one site is deteriorating faster then the other side, or the material gets beat up.
In order to enjoy your longboard for a long time, it is necessary to maintain it.
First you need to know all your parts: deck, trucks, bushings, bearings, wheels and what and how they are used for.
When you know this, you can maintain your board well.
Below are some video's and articles to help you with this.