In the old days (pre-2013 when FP was all you had, and not all of the "extra" attractions that had to add a FP+ line), you would usually run out of FP on the headliner attractions by early afternoon. In busy times at parks with few rides (think Epcot before Frozen, and the third theater at Soarin'), you could exhaust all of the Test Track fastpasses by noon, easily. Or look at DHS, before Star Wars opened, before there was Toy Story land or a third track at TSMM, you'd probably be out of FP for TSMM on a busy day by 11am.
The good news is that there are a lot more headliner attractions and capacity now, compared with 2013, so we're returning to the old FP model but with more capacity. The bad news (or maybe just an unknown factor) is that if guests can make a
Lightning Lane reservation starting at 7:00am, will all of the LLs get eaten up instantly for the headliners? That has the potential to be more like FP+ where the non-MK parks would have everyone flood the top 2 or 3 attractions. The only advantage is that
Genie+ and IA$ are paid options that not everyone is going to buy, whereas every guest got FP+ reservations to make. That's probably why Genie+ won't cover the top two attractions at each park, and you'll have to buy those slots as IA$ or wait in a (probably long) standby line. It will also depend on when you can make your next LL selection (two hours? three? depending on on-site vs. off-site?). If you log on at 8:30am and see LL for Haunted Mansion at 3:00pm, you'd have to think about it if it meant you couldn't make another LL selection until much later. I think that's really the big piece of info that Disney is still sitting on - what's the "timeout" until you can get a new LL reservation? And, of course, what's the final list of which attractions have Genie+, and which have IA$.
We'll all have to see how it plays out, but I could expect that most people will fire up the app when they wake up (probably right at 7am for a lot of folks, though you might sleep in if you're not aiming at the headliner attractions before evening time), pick their top LL choice for the park, maybe add an IA$, and head out. I could see us planning a MK day like this: wake up, buy an IA$ for 7DMT, reserve an LL for Peter Pan, enter MK and go right to Fantasyland, and ride other attractions (maybe give the standby line at 7DMT a go if we get there at rope drop), and then start touring "normally" after that initial burst in Fantasyland. Back in the pre-2013 days, you'd use the touring plans app with crowdsourced data to see what the FP return times were for various attractions, now you'll just log onto Genie+ and see that Haunted Mansion has a 1:30pm return time, so you grab that, and then head that direction. Or you see Little Mermaid with a 12:00pm return time and head that way instead.