Do you know how long it would take to synchronise data from sfdc to sfmc if there around are 1.5m contact records being synchronised?
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It will also depend on how many contact record fields and how much data in such fields that you wish to sync.
From experience, I synced similar volume of records and ~25 fields which took about 10 - 15 mins.
At the first time sync this is the master sync as from there on it becomes a delta sync of contact records which have changed