It depends on what you are measuring and on how steadily the changes are kept up. Some things, like sleep and energy, many people notice within a few weeks. Blood tests move more slowly.
HbA1c, for example, reflects roughly the last three months of blood sugar, so a repeat test is usually done after that gap. Cholesterol, liver and thyroid tests are also usually repeated after a few months, as your doctor advises.
Small changes that you can keep doing work better than strict plans that last two weeks. If a number has not moved after a steady few months, that is a good reason to talk to your doctor, not to give up.
This article is for general education. It does not replace advice from your own doctor.