From what I can gather, that can vary between people. Just like the amount of time it takes for us to go into ketosis can vary. There's a lot of posts about people struggling to get to ketosis, but I got there before the end of day 2 with little difficulty.
My thought would be that the harder you found it to get into ketosis then the less you want to risk messing about with any extra carbs, although I can't proove this, I suspect that you would go out of ketosis easier and then you would have to go through getting back there. If you do that to yourself then it isn't a lot of incentive to stay on this diet, the point is that after the first few days, then it's relatively easy not to eat.
I can top up without affecting ketosis. I've done so a few times, mainly on very high protein or very low carb stuff, but I did snack on something that contained nearly 20g of extra carbs with no ill effects. I won't say what I've eaten here, but i can do on the non TS board if you're interested. I do TS maybee 5 days out of 6, but I do have a tendency towards getting leg cramps, needing extra potassium, and there are low carb sources of that I can top up on.