If you intend on working on an isabel porcelain project, it will take at least two generations to get the lav to show on the buff.
F1 buff x lav would produce 100% Heterozygous (split) lav birds. Phenotypically, they would look as though they were black x buff. The birds would range from smutty buffs to almost black with gold in the hackles and saddles of males and on the breasts and hackles of females. F2 (breeding f1xf1) would produce 25% lav, 50% het lav, 25% non lav. The het lavs and non lavs will look identical in that they are not visually expressing the lav gene, so it would be impossible to determine which of those birds are carrying the gene. Breeding f1 back to the lav parent, you will get 50% lav, 50% het lav. The birds expressing the lav gene will look similar to many isabel porcelains.