Your benefits begin instantly. You may even get the extra mileage bonus for the flight you qualify on; I did when I made 2P and 1P, but I don't know how often that occurs.

If you have any existing itineraries, you'll need to call reservations to get them to remove and reenter your MP # to make them reflect your new status. This sometimes can't be done for a week or 10 days after your qualifying flight has posted, as MP has your new status (and will give you the bonuses, etc.), but reservations does not yet. They can even see that you have the status on their screens sometimes but not get the system to admit it's there, so be patient.

As for *G lounge access, some seem to be stricter on wanting the new card. YMMV there.