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Naps work best either short or complete. Under 20-25 minutes you stay in light sleep and wake refreshed; between 30 and 60 minutes you wake from slow-wave sleep feeling worse than before. A full 90 minute cycle avoids that entirely.
Nap timing
alarm = target nap length + time to fall asleep; target is 20 min or a full 90 min cycle
It lands you in deep slow-wave sleep at the moment the alarm goes off, producing 15-30 minutes of heavy sleep inertia.
Generally by early afternoon. Later naps eat into the sleep pressure that helps you fall asleep at night.