Bell fragments in fortresses respawn at random intervals, ranging from 3 to 24 hours after looting.
Look for the green spotlight from them.
When this happens the landscape around the fort and its structure will also revert back to original.

Do not put a portal too close or it will be gone.

Fragments required to craft special Ashlands items, typically limited to one per server, respawn randomly both in time and location.
The respawn interval for these fragments is between 16 to 32 hours.
Notifications for looting and respawning of these fragments will be broadcast in the chat for all players to see.

Be aware that the ballistas can respawn several times so you are fighting a stack of them at once - getting hit by 5 or so javelins at once can be a rude experience

Apart from the above respawn rates - the fortress inhabitants and ballistas will typically respawn in 2 hours and be ready to kill you.