P&I says it has to be more than 4 days, but no longer than 2 weeks. I haven't tried doing 3 days, but you could test it out. Also, you can't take an interruption until you have had 20 effective injections. P&I explains "effective" like this, "We distinguish between the first three injections, which we call “non-effective” as far as the loss of weight is concerned, and the subsequent injections given while the patient is dieting, which we call 'effective'." I think that's why you hear both 20 days and 23 days. It is 23 days of total injections, or 20 effective injections. Again, if you aren't quite to that many days, you could give it a try and see how it goes.
I think another 7-10 injections after the interruption is fine. I'd probably aim for 10 just to be safe.
I've done a planned interruption on most of my rounds. They work just fine for me. I eat P3-style, eating lots of fat, protein and non-starchy vegetables. I don't seem to gain as long as I eat enough fat/protein. I don't drink very often, so I don't have advice on that.
Here's what P&I says about interruptions:
"If an interruption of treatment lasting more than four days is necessary, the patient must increase his diet to at least 800 Calories by adding meat, eggs, cheese, and milk to his diet after the third day, as otherwise he will find himself so hungry and weak that he is unable to go about his usual occupation. If the interval lasts less than two weeks the patient can directly resume injections and the 500-Calorie diet, but if the interruption lasts longer he must again eat normally until he has had his third injection.
When a patient knows beforehand that he will have to travel and be absent for more than four days, it is always better to stop injections three days before he is due to leave so that he can have the three days of strict dieting which are necessary after the last injection at home. This saves him from the almost impossible task of having to arrange the 500 Calorie diet while en route, and he can thus enjoy a much greater dietary freedom from the day of his departure. Interruptions occurring before 20 effective injections have been given are most undesirable, because with less than that number of injections some weight is liable to be regained. After the 20th injection an unavoidable interruption is merely a loss of time."