I think the party usually starts at 7 but you can get in with your ticket earlier than that. I believe it goes until Midnight (or maybe that is just when we gave up).
This past year it was 7pm to midnight. We entered MK with our MNSSHP tickets a few minutes before 5pm. The park was clearing out and pretty empty.
Between 5:00 and 6:00 we did Peter Pan's Flight, Winnie the Pooh, Snow White's Scary Adventure, Mickey's Philharmagic and Dumbo. At 6pm everyone with a MNSSHP was directed to Tomorrowland so the CMs could set up for the party in the rest of the park.
At 6:00 we rode Astro Orbiter, then Space Mountain and by that time the whole park was open for MNSSHP.
We did A LOT between 5pm and when the party officially started. And, we stayed until midnight!