Even though Apocalypse may have Shin Megami Tensei IV in its name, it’s a full-length game that stands on its own and you don’t need to have played the original.
In proper Shin Megami Tensei style, Apocalypse is definitely not a short game. It has a lengthy story, that’s complemented by a host of challenge quests and optional activities. If you want to just blaze through the main story, which may be a little hard due to the difficulty, the game will probably take you 50-60 hours. My own playthrough clocked in at around 70 hours, although I spent a bit of time training and doing at least half of the challenge missions.
If you want the full experience out of Apocalypse, including side missions and keeping the game set on a harder difficulty, it may take you upwards of 80 hours to even 100 hours. There certainly isn’t a lack of content in Apocalypse, and you’ll spend plenty of time running around Tokyo trying to complete every quest and befriend every demon.