From an email I received earlier.
These types of trials have historical been locked to Plus subscribers but these seem to be open to everybody from what I can tell. As is usually the way, the trials essentially mean that you download the full game (SackBoy 31.42 GB + Death Stranding 69.26 GB) and the time starts from when you claim the trial and not from when you start playing it.
Obviously download speeds are going to affect the gameplay time but that's a pretty hefty lump of gameplay for both titles and it looks like you just unlock what you've downloaded by buying it if you then want to.
SackBoy is actually on sale now which I did post about a couple of days back.
I'm somebody who likes to take their time with games but I'm pretty sure that both the games could be finished if you claimed one at a time and wanted to rush them.