What's the natural lifespan of a Borg drone?

A Borg drone does use biological brain which can't be replaced, I think. Every machine including brain has a finite age.

For how long can a Borg drone live before dying naturally (death in battle is not natural). Does it depend upon the base species' lifespan? If yes, how long do human based Borg drones live? Or, are Borg drones immortals due to some kind of Borg tech?