Detraining is something that affects all fitness enthusiasts, but generally has more dramatic effects on highly trained athletes. The extent of the fitness drop off depends on how inactive the individual is. If the person is immobilised by an injury or forced into bed rest due to illness, then the loss in conditioning will be rapid. However if you are still managing to continue with your normal functional activities i.e. walking, stair climbing and lifting your detraining will be more gradual.
Other than speed, which can be maintained with one speed session every two weeks, most aspects of fitness will decline very quickly. VO2 max which is the best indicator of endurance has the potential to decrease by 14% in 2 weeks if the individual is completely immobilised. If you have stopped training, but are still doing your usual everyday tasks then you are likely to experience a 15% decline after 1-2 months. It takes 4-6 weeks for any significant strength losses to occur; however if a person is immobilised then the decline can be much more rapid, up to 5% a day. Flexibility can be lost very quickly and therefore should be worked on throughout the year.
To avoid detraining a particular aspect of fitness you should not neglect any area for longer than 4 weeks and if possible have a weekly maintenance session. For maintaining each training component intensity is more important than duration e.g. endurance work at an intensity greater than 70% of maximum heart rate, and for strength work you should look at performing repetitions to maximum.