I'm not sure about celeriac as i don't eat it, but cauliflower is pretty low carb, i believe a whole medium head of cauliflower is around 15g which is about what the max you should be having per day in terms of carbs from veg. Which is quite a lot of cauliflower. I know some people on here don't even count their veg at all - myself, i just serve myself a portion of it whenever i want but i don't really eat a lot of veg so no danger of me going over.
As to how long it takes to get back into ketosis after carbs, this depends on a lot of things. How much carbs you eat plays a huge role - if you went all out and ate 500g or more of carbs, especially refined carbs like white bread, pizza, etc, you will be back to square one, whereas if you just had a sneaky chip or two but have otherwise been good you might find you don't get out of ketosis hardly at all, anything from just an hour up to 12 hours or so. Also, activity plays a role in this - if you eat some extra carbs (lets say 30g extra) but are then super active (go to the gym) right away after eating then your body will demand the carb as fuel for your exercise before switching to it's normal ketone burning state. If you exercise a longer time after eating, then your body will burn glycogen stores from your muscles which will have built up after eating a high carb diet, also a good thing to help speed up ketosis as well as get rid of any extra water weight you might have accumulated from the carb load.
Eating one carb meal after another after another for 24-48 hours or more really does put you back to square one pretty much unless you have one of those great metabolisms / are super active (i doubt any of us have struck the gene-pool lottery on this one since we wouldn't have got fat in the first place!).
