Manufactures say the lifespan of an external hard drive should be at least 4 years if you use the external hard drive a lot.
Me personally I have an external hard drive that is 5 years old now and is working just fine but I don't use it very much except just to back up my website files so it doesn't get much use.
You should get at least 4 years of use out of the external hard drive though but you can expect much longer lifespans from the external hard drive depending on how much you use the external hard drive.