As with most things related to a system as complex as the human body, it depends. According to the ACSM, two weeks without exercise can lead to significant loss of cardiovascular fitness. Two to eight months of detraining can erase virtually all of your gains. As you detrain, cardiovascular fitness tends to decline first, with muscle strength declining later.
Other factors are your age, and your exercise history. If you’re younger, you’ll probably lose fitness at a slower rate than someone older. If you’ve been consistently exercising for a long time, or at a high intensity, your losses will probably be slower than for someone who just started.