it all depends on how far off type of a crossing you make. There's no set number. on the 4th generation you will be 15/16 back to pure, so that's pretty darn close. But I have outcrosses in various birds that were close to type to start with and some times 2 will have you pretty darn close. A long time, or tuff is in the eye of the beholder. For me 4-8 years of work is no big deal, to others 2-3 years is unheard of. On my bb red sumatras, I think they are at year 6 or so. With that you have a type correction, plus the head ache of keeping the color. So you have to do a year on type then back cross those chicks the following year to get your color back, then go back to the solid sumatra and do it again. So with a color cross like that you have to double the years needed due to the color you are working with. Same with any recessive genes too, takes a back crossing year to get them back if they arent present in the primary breed you are working on.
Black to blue or dun is easy though, half should be the color you want that way, select them, then just continue with the type correction...much quicker