The first ledger snapshot will take place anywhere between 0 seconds and 12 hours. You need to wait 12 hours to see how it behaves to make sure it has taken a ledger snapshot. (To be very precise: the first snapshot is taken after it sees 2160 (k parameter) blocks after your last snapshot. So the time it will take is about: (2160 - lastLedgerSnapshotBlockHeight) * 20 seconds, not counting for missed slots by pools)
Only after your first snapshot after node start will it start taking snapshot at configured snapshot interval. (with some side notes)