As the title- I know that 56 days is the general consensus- is that 56 days from mating or from ovulation?

Hard to say as until recently very few people would know when ovulation occurred, so I'd say 56 days from mating.

It is generally found that most bitches won't mate before the LH surge two days before ovulation, so could be 56 days from LH surge or and maybe 54 days from ovulation, which would be 52 days from conception.

But then we have no way of knowing if those pups surviving at 56 days were actually only conceived 56 days before birth, and those whelped 56 days from LH surge or ovulation would not survive.

Never done any testing, just gone by the dog. My earliest litter the bitch went into labour day 54 from first mating. Pups born after midnight so JUST day 55. Four pups, all fine. In fact good sized pups all of them.

Thanks for the replies.
My bitch was progesterone tested and ovulated on the Monday, was mated Monday and Wednesday.
She's now day 53 from the first mating (and ovulation) she's absolutely huge and dripping milk as well as being very unsettled, so it's just made me a little concerned.