Eons. Its probably not that hard to reach some random Dungeon on a random world that isn't the land. The difficulty lies in how extremely many worlds there are. In addition to the reality that gods can and have died in the labyrinth. Its a role of the dice whether you'll survive a dive or not. Even with respawns Richter and other chaos seeds aren't all that safe, should be plenty of creatures more powerful than the lich. Some of them might start by "licking off" respawns and similar goodies before devouring the adventurer.