The default despawn range for monsters in Tibia seems to be 50. This means that you can safely lure a monster at most 50 squares from its spawn point. Once it reaches 51 squares, it will despawn. The range is calculated as a square: as soon as either the x position or the y position of the monster is more than 50 from its spawn point, it despawns.
In this screenshot, you can see I've lured Owin from his spawn point of 33368, 31614 all the way south to 33375, 31664, which is 7 squares east and 50 squares south of his spawn. I took him one more step south and he despawned immediately.
This despawn range mechanic only applies to monsters that have a spawn home. Almost all creatures fit into this category, but there are not very many bosses like this. To name a few:
- Varying spawn bosses.
- We have had Mahatheb despawn on Nefera because someone lured him too far. The GT bosses (Burster, Dreadful Disruptor) and the Liberty Bay Hive bosses possibly have a despawn range, but their rooms are too small to test.
- The Archfoe Were-bosses.
- As demonstrated, Owin has a range of 50. I have also had Ayana despawn on me because I lured her more than 50 squares west, so it's likely the case that Ayana and Tamru are also 50.
- Ancient Lion Knight.
- This boss is a special case, he has been modified to have an exceptionally small range of just 5 squares.
A notable exception is Custodian. Despite being a regular spawn home boss, he can be lured all the way to the north of the basement, 146 squares:
Bosses that cannot despawn from luring are bosses with these spawn mechanics:
- Lever bosses.
- Bosses and creatures from raids. This includes almost all Nemesis bosses. To give some examples:
- It is common to lure the Midnight Panther all the way across the Tiquanda jungle, it will never despawn.
- The Old Widow surface raid can be lured all the to Venore gate.
- Zao surface bosses (Fleabringer, Zunzu, Flamecaller, Dreadmaw) can be lured as far as you want.
- Raid timer spawns.
- This is the spawn mechanic of Albino Dragons, Mawhawk, Bane Lords, and perhaps a few others. These are creatures/bosses that have a chance to spawn based on a periodic timer. For example, Mawhawk has a chance to spawn every 10 minutes, and Albino Dragons are every 30 minutes. I tested luring the PoI Albino Dragon as far east as possible. I was able to lure it 58 squares east of its spawn point, and it never despawned. I think we can be fairly confident that raid timer spawn monsters can't despawn from luring, but I wouldn't say this is conclusive evidence, because it's only one monster type, and only 58 squares.