Horses lose fitness after about 6 weeks. They can be totally stalled (lets stay), and pretty much still have the same muscle, bone, and heart rate fitness. Then they can come back and be ridden a couple times, and they will be at the same fitness.
Once fit they can be worked 1-2 times a week and still be in the same fitness condition.
Sad for us humans, we lose fitness within a few days. Horses, not so.
Here is an article, read all it, or scroll down to the subject of “detraining”. Intensity is your riding, whether it is lunging or hard hard work for the horse.
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/hrs6942
Here is more about fitness, which is where I got the article.
http://enduranceridestuff.com/blog/endurance-riding/
Even my current trail horse (I am not riding endurance any more) use the same principles I did when I endurance ride. Right now she is getting some hill work to strengthen her hind end. Also to get myself fit I use the same type training, although varied. Interval training.