It all depends on how much you use it. I have had Harbor Freight Floor Jacks for at least 10 years. 2 of them I bought at the same time and they work great. I use the crap out of them and I am not nice to them either!
On the other hand I bought a Costco Aluminum Floor jack and it lasted for 3 years. It got so bad I just threw it in my dumpster.
Again, depends on use. Sitting in the corner all polished up it will most likely last forever.