I tell my clients that they really are best off avoiding ALL fizzy, shop-bought drinks until they're firmly in ketosis - and that can take a few weeks. After that, if they want to risk getting knocked out of ketosis, then they should strictly limit themselves to half a glass every week or so. It's just not worth the risk of having to wrench yourself back into that lovely state of not feeling hungry.
I speak from experience - diet coke AND coke zero have knocked me out of ketosis before - just not funny!
xxx