Depends on how "easy" you rode for that month I guess. I had two weeks off the bike after a crash, started back riding with 3 weeks until time trial event. First week I did just moderate rides before getting back to structured training. My average power for the ITT was down about 25 watts from where my FTP had been. That's just under 10% drop.
Assuming you're not sick or anything it should come back pretty quickly, though.