Not long to get your yellow, maybe 3-4 months.
A bit longer for orange, maybe a 6-12 months (after yellow)
Green, maybe 1 - 2 years (after orange)
Longer again for blue, maybe another couple of years - 3 maybe
Long time for brown, 3-4 years - maybe longer. Blue seems to be where many just get stuck
Brown, well.... sometimes long, sometimes not. Mine was 2 years - probably others might be a bit longer. My coach's view is the "meat" of the learning is done in green/blue, brown is to refine it. Probably others would see it differently so maybe 3+ years?
Heck, it all depends on yourself & how hard you train & how much you compete. In Oz, you need points before advancing - gathered in competition. (10 for win by ippon against the same grade, 15 for 1 grade up, 7 for 1 grade down). I needed 150 points in 2 years for black.