Question about timing

My ceremony is going to be at 7:00pm - it is going to be just a ceremony not a mass but the woman said it will be about 45 min. long. Then my reception is going to be immediately following. It's walking distance from the church and I think many people will walk because it's in NYC and only about 3 blocks away. My question is should I tell the site that I want the reception to start at 8:00pm or 8:15 or 8:30. I don't want it to start too early, but I also don't want guests to arrive and not have anything to drink or anything.

Thanks!