It took me about 4 years of training seriously, although i had been training for another 6 before that. The first six years was mainly in kids classes, only once per week and never entered competitions or did randori, then i moved and joined a strong club were i started to take it seriously.
Honestly, in most western countries a black belt in judo is a high grade. If you go into a judo club in a european country and someone is wearing a black belt they should really be able to throw lower grades around if they want to, they should have decent newaza and they should certainly be technically sound and know all the common throws. Its not as low a rank as is sometimes sounds reading online forums. I personly see my judo black belt as a much much higher achievement than my bjj blue belt. Perhaps in terms of absolute skill a purple belt would be a better equivalent.
Its just my personal opinion but I believe it should take atleast 4 or 5 years of hard training and competing to reach the level of black belt.
On a side note, with the system in my country (UK) being what it is everything 5.dan or above is basically the same in my books. 5.dan is the highest belt you can fight for and win in contest here (along with doing technique demonstrations) and everything above it is entirely political.