Although the XP Bottle Agathion is no longer available for purchase in the L2 Store or L2 Galleria, this page serves to explain how the item works, for those players that still possess the item.
The following image refers to the 10 hour version of the XP Bottle Agathion, but the instructions and explanation pertain to the other two versions, 20 hours, and 30 hours, as well.
Note that the item description in-game currently says that 5 mana are consumed upon use. This is an error. The fact is that 10 mana is consumed upon use. This error will be fixed in the future and it has been announced widely to players so that one knows the conditions of the item before purchasing or using it.