Exactly how much you will lose depends on the length of your break and your fitness level. If you are consistent and exercise for 5 or 6 days a week then it takes more time to get out of shape. Even after taking a break for a week, you will manage to easily bounce back. This is because by working out consistently strengthens your muscle memory and even if you miss out your workout, your skeletal muscles may stick around for a long time even when other muscles shrink.
If you take a break because you are sick then you will start losing your strength within two or three weeks as illness will make you weak. However, if you are not sick then you will start losing your strength after four or five weeks. As per a 2000 study, young people lose 8 percent of their strength after 31 weeks of inactivity, while older people lose 14 percent of their strength during the same time.
Sadly, cardo loss is even quicker as compared to strength loss. As per a study conducted in 1984, 21 days of inactivity decreases the VO2 max by 7 percent and the endurance reduces by 50 percent. Cardio fitness might be the first to go, but it takes less time to regain it.