Rich - it doesn't have to be whole days! You can split it up into sections and do a different group of hours every day. Just need to be flexible with both what you eat and what time you eat it
The downside of going any length of time carb free (and let's face it cucumber and lettuce both contain carbs, it's simply because we don't usually eat 10 tons of either at a time, the amount of them is considered negligible!) is if the body then gets used to that, it will become more efficient at manufacturing the glucose it needs in order to function, from either or both protein or fat. It's simply easier and quicker for it to do from carbs, so it always deals with them first! If your body gets to that state where it uses protein/fat for its fuel - then it will then also need sufficient insulin to do so. You're then into a more complicated set of calculations than 'just' the carbs, cos neither need as much fast acting, one needs less than 50% and the other still less than that - aargh.
Protein and fat, plus above ground vegetables or fungus - ie mushrooms! - are the easy answer though, generally.
Could start the day with bacon and mushrooms, could lunch on eg meat and green salad (some of the 'mixed leaves' packs in supermarkets make the salad both look and taste a bit more interesting, even if you don't add a simple dressing of 'oil & vinegar' (nice enough as it is but with a very few extra tiddly bits of herbs etc, this quickly becomes the classic French vinaigrette and either livens up a plain looking bit of salad no end, especially if you toss the salad in a small amount of anything whether it's a teeny dribble of oil and a cursory shake of vinegar, or a smallish spoonful of mayo - don't need a ruddy great dollop of anything) and again meat and decent veg for dinner. Steak onions and mushrooms with something green?
Brekkie could also be as much as you want of full fat yoghurt - a lot of us like the thick Greek yoghurt. You could probably afford to chuck just a few berries in there - shedloads of lovely raspberries available just at the mo - but not a lot of them. Nobody said you need to starve!