A play session is not how long the game goes, but how long an “average player” is meant to spend on your game. If someone spends 15 minutes, will they explore everything the game has to offer? Will 30 minutes be enough? Or maybe 3 hours?
If you give the game to a friend of yours that has never played it before, how long until they stop playing?
It’s worth noting that the session time is not a strict measure, but more of a guide of what players to expect. Some players are looking to spend 5 minutes on their lunch break, others want to invest hours.