That all depends on a lot of things. How old is the horse? How sick was he? What kind of riding was he used to? What kind of riding do you intend to do (moderate intensity means different things to different people)? What is this horse's natural disposition?
I got back on my 7 year-old mare after a layoff of about 6 weeks and she acted like she had forgotten everything I have taught her over the past 6 months. It took about an hour for her to get back to normal and realize that she still has to mind me, even after a vacation. Of course, she is pretty green to begin with.
As for conditioning, as long as you are not doing something like Steeple Chase, he should be getting back up to speed within two or three days. Start a little slow, just to allow him to get focused again and allow him to get back into the swing of things, but I'd be working full-speed by the third day (if your "moderate intensity work" is similar to mine).