That really depends on the strains but it would take a good few generations of crossing checking the offspring to see if the male you use passes the traits u want onto the next generation.to do a cross then stabalise it takes alot of time,knowledge and effort.thats why most breeders only do f1,s and why theres so many shitty phenos in the seeds we get.
breeders like loompa do alot of breeding of 5/6 generations,not to bottle neck the line as his yeti f3 still retains alot of different phenos butmost importantly most of the undesirable phenos have been eliminated giving a line of seeds that,while still having a good bit of variation between phenos ,theyr all what i would consider "keeper quality".but your paying a premium for beans like that simply because it takes more time and effort to acheive which is cool but when they charge $200 for crap thats out of order.swamp boys have just been accused of this on firestax.which suprised me as i thought that crew knew what they were doin.but i suppose growing for bud and breeding arent the same thing.