Hostile mobs never despawn, filling the world as you play (specially underground or near spawners). It doesn't matter how far you go or how long it passes, the mobs are where "you left them".

After playing for several hours on the same world, underground caves are getting clogged and moving around the nether is getting sluggish near fortress because of the insane amount of blazes around.

They are also clearly saved with the game.

Please despawn like on PC version or this is going to escalate to unplayable worlds quickly.

Note: Playing on survival mode.