Laptop hardware lasts as long as you don't abuse it. They last as long as they can handle comfortably the operating system and the softwares that you want to run on them (e.g. too slow or simply isn't compatible anymore).
My personal rule of thumb re laptops is to buy the highest model (i.e. fastest processor, largest harddrive, most memory, etc.) that you can afford. Future proof it since, unlike desktops, you can just easily swap a new processor or video card in there when time comes for an upgrade.