I took 3 months off last year (Sept-Dec) due to patellar tendonitis (and some burnout) with either no riding or z1. My FTP went from 280 to 220 but I got back up with 240 within 2 weeks and I'm probably sitting around 265 right now. The other big thing is the weight. I gained 20 pounds and still have about 10 more to lose. From a Power / Weight perspective, I was at 4.3 w/kg, went down to 3.4, and right now and somewhere around 3.85. For me, a guy that's really only good at long efforts and climbing that's a lot.
Taking 2 weeks off would hurt as well, I don't think you lose muscle just some aerobic capacity.