Muscle and Strength

If you are unable to get out of bed (ill or injured in hospital) or during a really sedentary holiday, you would first observe reductions in the “skill” of the movement, then your power, then strength, and then your muscle across time. Bone mineral density loss can also occur as soon as 1-week of complete bed rest https://pubmed.ncbi.nlm.nih.gov/6339154/ or 2% of your muscle mass reduction per day in this study of the critically ill people in hospital https://pubmed.ncbi.nlm.nih.gov/36597123/. However, assuming you are not subjected to complete bed rest and really ill, and are just taking a break or going on a holiday, then your age, previous strength training experience and how active you are on your break dictate the extent of changes to your strength and muscle.

Participants of strength training who start training at an older age (the statistical cutoff for this in research on average is around 60 years old) have quicker losses than those the same age who have been strength training for a while. So “old adults should follow a long-term and systematic routine of exercise throughout life, in order to improve and maintain their physical functions and to ameliorate their life status” https://pubmed.ncbi.nlm.nih.gov/20440099/ 

However, this does not mean your age should be the first factor to blame if you feel any reductions in exercise tolerance. Your lifestyle changes, such as more time commitments for work or family or illness that occur over your life, have greater effects on reducing exercise tolerance than age itself. An “active” life can greatly reduce the loss of power and strength associated with aging and reduces the extent of change in exercise adaptations during breaks or holidays Peak anaerobic power in master athletes - PubMed (nih.gov) It is hard to separate the effects of age from the effects of a sedentary lifestyle in scientific studies so take that into account when an age as a statistical cutoff is mentioned as a mediator of changes throughout this article. 

Independent of age, the smaller your total resistance training experience or the less time you have completed strength, muscle building or power training, the quicker you reduce these attributes in general. For example, if you have never completed resistance training and then start for 12 weeks and then rest for 12 weeks, you'll reduce about half of the strength and less than half of the muscle you have increased in size during the previous 12 weeks of strength training. However, you will regain all these attributes and more in half that time when you restart strength training again. https://pubmed.ncbi.nlm.nih.gov/32017951/ 

Expect less changes if you have been strength training for longer than a year. So yes, the less time you have trained, the quicker your gains reduce, but it still takes many weeks to reduce appreciable strength and muscle, and you regain your lost gains faster when restarting than how long it took to develop them in the first place. An analogy for why regaining muscle and strength occurs more quickly after a rest than when improving them in the first place is thinking of scaffolding vs building materials when building a house. When a house is first being made, scaffolding is put up to make applying the building material easier. The longer you train, the more likely your muscles and nervous system will develop its kind of scaffolding (the muscle version of scaffolding is called the “myonuclear domain” Human Skeletal Muscle Possesses an Epigenetic Memory of Hypertrophy - PubMed (nih.gov) If rest occurs across weeks to months, the building material is removed (you might notice your muscles get smaller) as the body needs lots of energy to have muscle and the lack of signaling from resistance exercise causes your body to de-adapt to save energy. However, the muscle's scaffolding is left behind, so when returning to training, your body rebuilds quicker as scaffolding is already present. Effects of training, detraining, and retraining on strength, hypertrophy, and myonuclear number in human skeletal muscle - PubMed (nih.gov)

So, if you have been training for longer than six months, an active holiday will likely not reduce your muscle significantly, apart from the reduction in skill and some strength required to complete a challenging strength exercise, and skill can be regained quickly (in weeks) when resuming regular practice again. The more active your holiday is, or if you can sneak in any bodyweight or hotel-based strength training, the fewer changes you will experience, but don’t be concerned about large changes.