In "Collective," Seven said:
"When I was first captured by the Borg . . . I was young and frightened. I watched my parents assimilated. Then I was placed in a maturation chamber, and the hive mind began to restructure my synaptic pathways--purge my individuality. When I emerged
five years later, the turmoil of my forced assimilation had been replaced with order."
The answer to the OP's question is: 5 years.
As for consistency? This is Voyager. It's never consistent.
We're never told if that's the case for all drones or not. Other issues might come up that prevent a drone from maturing at the normal rate. Energy shortages, enemy attacks etc.
I'm sure the maturation process was a second priority when Species 8472 began their attack.
The maturation process would likely vary between species that have a different speed at which they mature as non-Borg.
For example, Naomi Wildman is a half-K'Tarian, and she was maturing twice as fast compared to a human.
It is possible that if Naomi was assimilated and placed in a maturation chamber, she would emerge from it 2.5 years later.
The nanoprobes likely increase the rate at which biological individuals mature standardly.
This factor at which the maturation process is increased would likely be the same for all species (if we go by Annika, then double, as a lot of humans reach physical maturity at the age of 16, and she spent only 5 years in a maturation chamber ... she was 11 at the time she emerged and by 5 years younger from a chronological point of view).
But it also depends at which age they were assimilated to begin with.
If Annika was assimilated at the age of 8 instead of 6, she would likely emerge from the maturation chamber earlier.
If she was assimilated at the age of 4, she might have been longer in the chamber.
And let's not forget that she was a drone for 18 years ... 13 of which were active (outside of the maturation chamber) ... meaning that there is a possibility the Borg improved the maturation process by then and even further shortened the time needed for an individual to mature.
It's in the Borg's interest to increase the speed of the maturation process so they have working drones as fast as possible, but the time for each drone spent in a maturation chamber would vary despite how you look at it as they were assimilated at various stages of their childhood and different species have different speeds at which they mature as non-Borg.