If the fans are constantly blocked, it would probably last around 6 months. If it's always on a bed, depending on the make, the amount of use, and a couple of other variables - 6 months to a year or two. - That's an estimate based on what I've seen.
Laptop is a misnomer. They are not designed to be put on beds and laps - the majority of models intake air from underneither - if this is blocked you may find the laptop will get very hot, and shutdown after a few hours of use. - Especially heavy processor use.
It depends on exactly how much you use it, and where, and the design of it etc. But you can significantly shorten the life of it.