Horses do not lose their conditioning as fast as humans. In fact, they hold their condition quite well. That's part of the reason big endurance riders will do a ride each month and then not ride the horses in between - the rides ARE the conditioning, and anything more than a light ride in between would be over-conditioning.
As for holding fitness over time off, it doesn't take too long to get a previously fit horse back into shape. If it did, many of my Idaho friends wouldn't be able to compete more than a few months of the year. One experienced friend in particular actually had to give one of her Arabians 2 years off, but was able to successfully compete in a 50 after just a few weeks of conditioning (her horses are turned out 24/7 as well). That's one extreme example, but the principle still holds true. Just pay attention to what your horse needs, but it shouldn't take more than a month or so to leg them up in the spring if they were fit in the fall and have only had a few months off.