You can measure ketones at any time of the day. However, ketone levels tend to vary, being highest in the morning and right after dinner [1].
It is also best to avoid measuring your ketone levels right after exercise because the numbers you would get then won't represent your mean ketone levels.
Being dehydrated, over-hydrated, and taking certain medication (Mesna) can lead to inaccurate results, especially with urine ketone strips [2, 3].
