How many days can you take off before losing noticeable fitness?

The over/under is probably the 10 day mark. Source: taken breaks throughout two decades of running consistently and a week tends to be fine, two weeks you tend to notice a drop off, split the difference.

You can take off pi + e = 5.86 days before it becomes noticeable.

How much time?

Can you still cross train?

Unclear how much time. No cross training for now.

I would say between 14-21 days.

From my personal experience as a competitive NAIA distance runner, I was involved in a auto accident in the summer leading into my sophomore season. During pre-season/early xc season, I would get random flare ups in my lower back and I would have to stop where I was and gingerly walk back. Usually I was out for 2-4 days, with 3 times where I was out for 14, 17, and 21 days each. I felt my fitness was okay up until 14 days, then I noticed definite aerobic loss. At 21 days, I noticed actual fitness loss.

I later learned it was an L3-L4 nerve compression, which required lamenectomy almost 10 years later. Needless to say, I redshirted and scrapped my entire sophomore year, xc & track, and discover pool running and core building.

I was never able to maintain a high level of fitness after my accident, although I ran PRs at 800 and 3000 steeplechase after recovery. But I never PR'd at the flat 1500 or above. My focus was on the 1500.

You just put those together randomly instead of deriving the appropriate equation.

It’s days = sqrt(pi^pi + e^phi)

Days? Weeks? Months? Approximately?

You will loose conditioning gradually up to about two weeks. By three weeks conditioning will be more significant and require longer recoveries.

When you come back to regular training and run a race, goals may or may not need to be adjusted.

This AI note is very similar to what I learned in an exercise science class decades ago. I remember A running friend telling me and I agreed that he didn't think the loss would be so high after two weeks.
AI Overview
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For runners, noticeable declines in cardiovascular fitness and endurance typically start after about two weeks of inactivity, though some studies suggest even shorter periods can lead to a slight decrease in fitness.
Here's a more detailed breakdown:
Initial Fitness Loss:
Within the first few days (3-5 days), runners might experience a feeling of decreased fitness and confidence, but key cardiovascular measures remain relatively stable.
After about two weeks, cardiovascular fitness and endurance start to decline.
Studies show that after 12 days, levels of enzymes in the blood associated with endurance performance can decrease by 50%, and VO2 max can drop by 7%.
Factors Influencing Fitness Loss:
Training Consistency: Runners who have maintained a consistent training schedule for several months will likely retain more fitness during a break than those who haven't built up a solid base.
Type of Activity: If a break from running is due to injury, it's important to maintain some form of activity to prevent complete loss of fitness.
Intensity of Training: Drastically reducing the intensity of workouts can also lead to a decrease in cardiovascular fitness.
Regaining Fitness:
Many of the detraining effects can be reversed relatively quickly when you return to training, especially if you have a good base of fitness.
It takes about two days of retraining to regain the fitness lost for every single day of training skipped.
After 2 weeks of not training significant reductions in fitness begin to occur, and you'll have about 2 to 8 weeks of training depending on the length of inactivity ahead of you to get back to your previous level of fitness.

The losses happen at different rates. Plasma volume drops in a few days. Tendons and ligaments are slow to change. I believe both ways. There's "detraining" in some studies.

It’s similar to losing your Summer tan. A few weeks.

I used to notice a difference after 3 days

Jack Daniels said 5 days.

I notice it after 2 days - I find it's usually my body forgetting how to run rather than actually losing fitness

A half day at most. That's why doubles are preferable.

Jack Farrell also said 48 hours.

Under 30, 2 weeks

Over 40, good luck

I noticed that kind of loss of neuromuscular coordination when I was in high school and college but less so now after decades of running consistently.