15-25 years. The ITF organisation has official minimum training periods between ranks, so 1st dan to 2nd dan is about 18months I think, however when you get to 3rd dan, you're training for about 5 years before youre 4th dan
I agree about 10-18 years after your 1st Dan, contiuned practice maybe 10-12 years. It took me 20 years. I ventured out and came back, after training in various arts, I attained higher rank then my base. Rank can become quite politcal. But thats another story.
Depends if you stay with the same dojo/style. Recieved my third dan in Tang Soo Do(Korean)way back and when I switched to Japanese my Sensei busted me to shodan 1st dan and after 12 years with him I'm now a 4th dan. It all depend how much they respect your previous training, attitude and apply it toward your present rank. It's a Sensei call... Some people ask can't you add the two together and call yourself a 7th dan......no, it doesn't work that way.