Fortresses respawn at random intervals, ranging from 3 to 24 hours.
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
Note the above respawn is for bell fragments - the fortresses themselves will typically reforn in 2 hours and be ready to kill you.