for Stellar Blade, ~25-30 hours to just do the main story, ~60-70 hours to 100% and platinum the game - you must finish the game in NG+ for plat, but it takes around 6-8 hours if you just do main story and skip all side quests. There is a trick to 100% the game with just two playthroughs (instead of 3) but i don't remember what it is, something to do with using PS+ cloud near the end to make two different choices.
I finished a few weeks ago and really enjoyed it, honestly my favorite game of the past 6-8 months. The visuals, hot NPCs, and combat is really fantastic and once you practice and get good at it, it tends to be relatively easy since boss fights are telegraphed with only a few simple moves. Its way, way, way easier than elden ring, dark souls, god of war, etc.
Trophies are not tied to difficulty at all so you can finish the game at any difficulty.