Yeh, you could eat tuna and boiled eggs all day long,... 5000 calories worth if you liked and you still wouldn't get out of ketosis... these two foods are about as carb free as it gets, particularly the eggs.
I think you'd have to have a couple of slices of bread, pasta, crisps, that kind of thing. It also depends on how active you are, exercise burns carbs super fast - it is interesting to read bodybuilding forums at times (and i have no real interest in body buliding, lol, i hate it), because they discuss nutrition a lot. A lot of them use ketosis generating diets to get lean and to build muscle. They all tend to give themselves a carb-loading just before they do a huge workout to feed their muscles, knowing that after their mammoth weight lifting session all those carbs will have been burnt by their muscles and they remain in ketosis to continue burning fat (one of the dangers of ketosis is if you overdo the exercise your body may end up breaking down muscle for energy rather than just fat if it cannot get enough energy fast enough, which is why they tell you on a lot of vlcd's not to go overboard on exercise - and also why bodybuilders fuel themselves with carbs immediately before a workout)