For TSL, APs could register for one of two early morning slots (7 -8am and 8-9am) on certain days throughout September. Only one date and one 1-hour slot could be selected (in reality, however, an AP that registered for a particular day could attend both slots on that day - it wasn't policed or restricted). Only a few hundred APs could register for the slots on any particular day. Not to stoke expectations, but the TSL event was one of the best events I've ever done at Disney. There was practically no one in TSL. The best metric I can provide is 14 rides on Slinky Dog in 90 minutes - something I don't ever expect to be able to repeat.
This event will have much larger AP numbers and the slots will last longer - but it's a bigger land with more to do and experience. Best guess is that it will be a very enjoyable experience crowd-wise and sort of spoil it for subsequent, crowded visits (but these are the risks we willingly take).