A very long time ago, I took all the commercial exams in one sitting. It was the last sitting of the UK national papers. I studied for a month and passed ( well) all but two. No question banks in those days. Two groups ( nav and tech), sat simultaneously over a few days. Finding myself now looking at the EASA stuff, I am sure this wouldn't be a wise strategy, but theoretically possible. If one has the equivalent grade of licence then no signoff required. If one really does know the stuff because of prior experience and can pass the coursework/ convince the training organisation to provide the sign-offs and take individual brushup, the exams are approximately every fortnight. Probably the minimum path now would be three sittings a fortnight apart.
Has anyone on here with a UK issued licence used the polish/hungarian/latvian exam route? Would love to know more details !