You can scan any time, day or night. The one rule is the temperature difference.
The 10-degree rule
You need at least 10°F difference between inside and outside for the thermal camera to pick up insulation gaps and air leaks.
The bigger the gap, the clearer the results.
Best windows during the year
Winter: almost any time works, the gap is usually plenty.
Summer: early morning or late evening, when AC is running but outside is cooler.
Spring/Fall: early morning right after a cool night, or late evening, gives you the cleanest results.
Check your thermostat and a quick weather app before starting. If it's a 5°F day, the scan won't pick up much.