The mass of the supermassive black hole in the centre of our galaxy is
4,100,000 M☉ = 4.1 × 106 M☉ = 8.2 × 1036 kg
Assuming the average mass of stellar black holes to be around 10 times the mass of the sun (M☉ = 1.9 × 1030 kg), and having about 100 million of them in the Milky Way, the total mass of stellar black holes in the Milky Way turns out as
1,000,000,000 M☉ = 100,000,000 × 10 M☉ = 109 M☉
That gives a total mass of black holes in the Milky Way of
1,004,100,000 M☉
There are only 1 to 5 new stars every year, within the Milky Way. Only one out of 1000 is heavy enough to become a black hole. So we might not have even one new black hole in 100 years. We can therefore ignore the formation of new stellar black holes, in our calculation.
The Milky Way is almost as old as the universe, namely 13,600,000,000 years old (the universe is only about 200 million years older). Supermassive black holes have a mass proportional to size and age of a galaxy. We can therefore find its growth rate by just dividing its mass by its age, assuming the age of the supermassive black hole to be the same with that of the galaxy. We thereby get
0.0003014705882353 M☉/yr (solar masses a year)
That is an increasing factor of
0.0000000000735294118 ~ 7.3 × 10-11