My vet has always advocated 6 months. I've been with them for 20+ years. However, on discussion with breeders, friends and a bit of research I have always opted to wait until 3m after 1st season. Had a few discussions with my vet on this and he does go down the line of less serious op etc. However also admits the heightened incontinence risk. I have two friends with incontinent bitches one done at 7 months one done after a pyometra at 2 years. I fully intend to wait with Bess so feel another long 'discussion' with vet coming up