Twice to 4 times a week though you are supposed to separate the days throughout the week.
Exercise needs vary on individuals for many reasons. First of all, it could be physical problems like weight. If a person is to heavy, or to small. Then starting out exercise slow is best. Often times over exercising for obese people, or people who are to thin, can cause blackouts, a loss of water retention, and exhaustion.
Another could be health issues. People with certain health issue may find it hard to work out, and need help with finding what works best for them.
Oxygen in the air can cause sliced fruit to brown, a process called enzymic browning (an oxidation reaction). Phenols and the enzyme phenolase are found in the cells of the apple, and when these are exposed to oxygen in the air, for example through slicing, the oxygen causes a reaction.