This is a question that I have searched the internet for many times and have never found a satisfactory answer. In my experience, losses to fitness come very quickly, but different aspects lose ground faster than others. I will say that these observations are based on my personal experiences, but I have also found other anecdotal data that confirms my thoughts. Sorry to the cold, hard science folks.
Muscle and Strength Loss
In my experience, muscle loss is the slowest process among the fitness indicators. With a sufficient caloric intake, I have been able to go up to two or three months and not visibly lose any muscle. Of course, this is not advisable, as other markers of fitness will quickly decline. Muscle seems to be very resilient. If your body has gone to great lengths creating it, it seems to go to great lengths to keep it. Building muscle is a very long and hard process. Now, I’m not sure why anyone would ever need to spend two months outside of the gym (I was taking time to focus on cardio), so that week long trip at a hotel with no gym is not likely to be devastating for your muscle mass.
Strength on the other hand is a measure of your muscle’s efficiency. Powerlifters know that in the week before a meet, you taper (or drop intensity) of your workouts. Obviously this is to allow your muscles to super compensate and get their strongest right before the big lift. When I am doing dedicated strength training, I find that strength levels fall off within about three to four weeks. Once again, you are hopefully still training in some capacity so that when you get back to strength training you come back with an increased work capacity.
Muscle endurance, or work capacity (when talking about lifting), is exactly what it sounds like. How much work your muscles can do in a given period of time with a given amount of rest. Work capacity losses in my estimation are very fast. Holding glycogen levels equal, a lack of training work capacity (usually high volume training), can cause fitness losses within a week and a half to two weeks. I find that if I stop training with high volume for two weeks, my first sessions back at it are very tiring, and recovery takes two to three times as long as it normally does.
Muscular endurance training (high volume) also seems to be the most sensitive to over-training. When we increase the amount of lifting (work) we do, we are increasing the amount of stress we place on our bodies. Bigger and stronger people doubly so. Dramatic volume increases, more than maybe three sets per body part per week, seem to be very stressful on my body. Work capacity needs to be gently increased over time, interspersed every four to five weeks with a deload.
Cardio fitness definitely declines the fastest after a lack of training. Cardio fitness is the slowest to build, but also has the highest capacity for improvement, especially as we age. My experience has been that levels of cardio fitness begin to fall off within three to five days of no training. Of course, if you are lifting weights, you are doing a kind of cardio, but not one that would give you the same benefits as low intensity steady state cardio. Having a high level of cardio fitness necessitates that you train with higher frequency and duration. Just ask a Tour de France rider how many hours they spend on the bike each week. Most health and fitness organizations recommend three days of cardiovascular exercise each week. I imagine this is because the compounded benefits of cardio can quickly dissipate if frequency is not high enough. On the other hand, lifting can be done as infrequently as once every other week (read the book Body by Science) and still post impressive benefits.
No matter which type of fitness you are trying to preserve, the most important thing is consistency. Hopefully my experiences have shed some insight on how long it takes to lose fitness. Thanks for reading!