After some time out of the gym, how long does it take to lose my fitness levels?

Well I'd say if you're a highly trained athlete then you can lose your top level of fitness quite quickly, say around two weeks, due to you're body conditioning to the training environment that you're in and that you get used to.

Not many people are training full time at the highest levels, so to maintain a level of fitness and stop for a while it all depends on your age and by "stopping" what you actually mean, if, for whatever reason, you have to stop totally and cannot move or exercise whatsoever then I'd say, again maybe 2/3 weeks you'll lose your fitness levels, but if you still keep reasonably active and raise your pulse rate a few times during the week I'd say it'd take a couple of months to totally lose all your fitness.

As with everything it's a little and often that always wins the race, take the hair and the tortoise example...