Fitness holds for about a week, then slips: plasma volume falls after five days, and VO2 max drops roughly 4 to 5 percent by two weeks.
The first days cost nothing measurable - VO2 max and cardiac output hold, and many return fitter as the body absorbs prior training. Weeks of full stop bring deeper changes in detraining studies: shrinking heart muscle, falling lactate threshold and glycogen stores, reduced insulin sensitivity. Caveats: the data comes from trained athletes, strength fades slower than cardio, and regaining is faster than first-time building. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
How long does it take to get out of shape?
Slower than you fear for the first week, faster than you want after that. The timeline from detraining research: three to five days off costs almost nothing measurable - VO2 max and cardiac output hold, and many people come back fitter because the body finishes absorbing the training. Past about five days, blood plasma volume starts falling, which cuts cardiac output; by around ten days VO2 max starts dropping, roughly 4 to 5 percent by two weeks. Weeks of full stop bring deeper changes: reviews of detraining in endurance athletes document shrinking heart muscle and stroke volume, falling lactate threshold and muscle glycogen, and reduced insulin sensitivity. Three honest caveats: the numbers come from trained athletes, so a casual exerciser's curve is shallower; strength fades far slower than cardio fitness; and everything lost is re-gainable faster than it was first built. The practical floor: one or two sessions a week - even at reduced volume - blunts most of the decline. A holiday is free; a month needs a maintenance habit.