The incubation period is about 5 days, i.e. you get infected from someone else about 5 days before you get symptoms. You can also be infectious for 2 days before the symptoms start, i.e. within that incubation period.
If you test positive for COVID-19 (Lateral Flow Test (LFT), or PCR), you should:
This starts from the day after you did the test. This is Day 1.
Some employers will want to see two consecutive negative LFT tests, on Days 5 and 6, before you go back on Day 6. It does not help if you cause an outbreak at work and many others are then off.
If a child or person aged 18 or under tests positive for COVID-19, they should try to stay at home and avoid contact with other people for 3 days. This also starts from the day after they did the test.
We have answered the question do I have to isolate now if I have COVID? The short answer is yes.