Every month extra dimensions reset (regeneration of all terrain) on the first day of each new month for convenience and server performance. (At 0:00 precisely for NA and EU servers respectively). The End resets every server restart, typically occurring every 2-3 hours. The restart schedule can be found here. The End is the only world which resets frequently.
“Extra dimension” is the term given to any world which isn’t the build world, such as The Nether and The End, The Mining World and personal Multiverse Worlds, or modded worlds such as Galacticraft planets or The Twilight Forest.
Not every dimensions resets in this way. Dimensions are excluded from this schedule include:
Personal Multiverse worlds
Compact Machine worlds
AE2 Spatial IO worlds
Galacticraft worlds (all space stations and planets)
The Last Millenium
Advent of Ascension worlds
Divine RPG worlds
Please note that no refunds will be given for any items left in these dimensions after they are reset. To be safe please ensure you have removed all items on the day before the deadline as to not be negatively affected by this process.
In summary, the End resets frequently, other worlds reset monthly, and some worlds don’t reset.