It really depends.
I say you could get up to two+ years. Most likely no more than 3. However, most hamsters nowadays come from large breeding mills that breed for quantity, not quality. So the genetics are really bad and some hamsters die randomly without a known cause. I say the average amongst hamster owners would be 1 1/2 year or so.
You can, however; buy a hamster from an ethically sourced breeder in which case you will hopefully surpass a year, maybe getting up to 2-3 years.
And I believe roborovski's are the species that live longest on average. I say they are more likely to get to 3 years than any other hamster.
Good care can certainly provide better health, but it's not going to fix genetics. I know numerous owners who adopted their ham, and the hamster died before even reaching 1. The hams originally came from pet stores, hence breeding mills. I also know someone who's long-haired syrian lived to be 2years 9 months. So pretty old for a pet hamster. (And he did come from a petstore) Though he did have lots of problems in his life and they spent lots of money on vet care.
My hamster is around 1 year and 3-4months. She originally came from a PetStore, yet I adopted her from a friend. She's a roborovski, but I worry that she could die at any given moment in time due to her genetics.