I managed to get the monsters to level 540 if it's indicative of anything. There's like nothing to prevent you to go far because at this point the only threat are the monsters that can kill you in one hit and even then the fountains are pretty much everywhere.

If you attempt to go far into the wilderness, then I suggest you max out your flight ability.

It may be indicative of hardware limitations or something else. My game quit making monsters altogether after I saw a giant clown monster at level 500 something and decided to land to kill it. A giant dragin and another giant clown came and I made quick work of them with the draginslayer sword...but then there was nothing. The game quit generating monsters altogether even when I flew back to the graveyard nothing spawned. Not sure what happened. One fella claimed to reach monsters up to level 1200 something; stating that the monsters then were so big they were unavoidable and one hit killed him.