Tim Downie wrote:
How long can I expect my fan over motor to last now that it's started to
make grumbling noises? Is it going to die soon or can they grumble for
years?

Fans can last for years in this state, or die fairly soon.
I find that opening these sort of things up, and adding a tiny, tiny
smear of grease helps for 6 months to a few years.
Pay attention to sealing of the bearing, if it had tape or something
over it, clean scrupulously, before replacing similar tape.

High temperature grease is probably easiest to find in your local car shop.
A little goes a long way.