As a "young master", I took five months off at the end of last year and after five months I'm still not back to where I was. Threshold is still 10% lower than this time last year. Of course, I still haven't cracked 9 hours in a single week, either, which I think is the single biggest inhibitor. I'm thinking a 3-4 week block of 10-12 hours a week would likely kick things into gear
But I agree with year round fitness. Once you have that it can be maintained with lower hours decently enough (for me at least). But it seems it takes a bit more to get up to that level if you do take time off. In the last few years I hadn't taken off more than 2-3 weeks at a time and it only seemed to take 2-3 weeks back to get to where I was. So much easier and less frustrating!