Minecraft wiki entry for Experience_farming, associated text: If continuous fueling is not practical, it is possible to set the intended yield of the farm by the amount of fuel loaded into it -- for example, a cactus smelter loaded with 70 kelp blocks will produce enough experience to bring a player from level 0 to level 30 -- with a single furnace, this will take most of four real-time hours (3h 52.5m). Note that this will be significantly slower than fishing, but where fishing requires the player's presence, the smelters can be left to operate on their own (in any loaded chunks).