Each country that requires a negative COVID-19 test for entry has slightly different rules and regulations for how long your test will be valid.
For example, some countries require that your test was done within 72 hours (3 days) before you land there, while others require that your test was done within 72 hours before you depart the UK.
We recommend taking your test as early as you can within the time period the country you're travelling to requires, as this will give you the time to get your test results back and make any alternative arrangements if you need to.